One of the oldest subgenres of science fiction is the Alien Invasion. Monsters from another planet come to Earth to eat our brains/mine our planet/terraform and colonize the Earth. If you're looking to write an alien invasion story, keep reading.
Necessary TropesMilitary and Warfare Tropes are useful when writing about an invasion. Alien Tropes are the other half of it.
Bare necessities from the first group: Earth Is a Battlefield, of course. If you want to make your story big, go for Apocalypse Wow and Big Badass Battle Sequence as the beginning and/or finale. If you aim for shorter story, try Easily Thwarted Alien Invasion. Remember that in such conflict, humans will have Home Field Advantage. At some point, however, they might feel like they're fighting a Hopeless War. If USA is part of your story, then of course it'll turn into Invaded States of America. Unless you set your story in the countryside, much of it will be Urban Warfare. Humanity's first line of defense from invader from above are likely to be fighter planes, so be sure to include Fighter-Launching Sequence. Sensor Suspense works both in air and on the ground. For extra awesome points you can have Epic Ship-on-Ship Action, Old-School Dogfighting and Epic Tank-on-Tank Action between human and alien tech.
Aliens are probably going to use make use of their higher ground (air?) and so use Orbital Bombardment. You need to shock your readers and underscore the danger - Monumental Damage and Shocking Defeat Legacy are your friends, as is Curb-Stomp Battle. Soldiers are likely to be Nicknaming the Enemy, so make up something earthly for the aliens.
Your heroes may end up Trapped Behind Enemy Lines, adding to the tension. On the other hand, you can go for Militaries Are Useless and have civilian heroes form La Résistance on alien-occupied territories.
As for the aliens, their appearance depends greatly on the atmosphere you're going for, so take a good long look at first section of Alien Tropes and ask yourself what would fit. As to other things, they invade us, so unless you want something more nuanced, Aliens Are Bastards. If they speak, they may claim The Right of a Superior Species. Particularly devious (but also redeemable ones) are Aliens Speaking English, although they may just as well be Inscrutable Aliens, who are creepier, because Nothing Is Scarier. Their materials are Not of This Earth. Bizarre Alien Biology and Bizarre Alien Psychology should be in full effect. The rest is covered further in below sections.
You'll want to put some thought as to the scope of your story. How many aliens are arriving? Stories have revolved around entire armies (Battle: Los Angeles) and sole survivors (E.T.). Additionally, there's the question of how much of the invasion you're going to depict. The Red Dawn films may involve a large-scale invasion of American soil by foreign powers, but mostly focus on one cell of The Resistance, with little news about the rest of the country getting in or out.
What are your aliens like? And, perhaps more pertinently, why are they here? The thing about invasions is that they aren't done for shits and giggles. Basic military strategy: you invade and occupy a region because it has something you want. If aliens are coming here, from godknowshowmany light-years away, it's because there is something valuable on Earth. What is it? Why is it valuable to them? That's going to dictate a lot of things, like their military and occupational strategy, and intersect with the idea of "alien" (IE superior) "technology" in a lot of interesting ways. For instance, if the aliens are Planet Looters and just here for our water, there's no reason they necessarily have to mount an armed invasion at all: just park a giant tanker ship in the middle of the ocean and turn on the vacuum hose. The first thing any humans would know about it is when the sea level started dropping, and it would do so in such a gradual fashion that it might take years for anyone to notice. (Observe how long it's taken us to notice the sea level rising due to climate change. It's been so gradual that some humans still deny that any such thing is happening.)
What kind of aliens are we talking about? Rubber-Forehead Aliens are played out, but Starfish Aliens can be impossible to write. Obviously, you'll want to strike a balance between the two, but what is your exact balance? What is the biology of these aliens? There are many, many amusing little twists of physiology you could play. Ever messed around with the chirality of proteins? Mass Effect did, to very interesting effect.
A really interesting question: when is this going to happen? Traditionally these stories fall into the Possible War category, taking place in the Present Day or 20 Minutes into the Future (Ender's Game), but what about the past? We've already had Cowboys & Aliens. How about Knights In Shining Armor And Aliens? Cavemen and Aliens? Wooden Ships and Iron Men And Aliens? Standard Fantasy Races And Aliens? Plenty of schlocky action movies, and maybe even some good stories, lie waiting to be discovered here.
There are questions of tone to consider. An Alien Invasion story is science-fiction, which makes it subject to the Mohs Scale of Science Fiction Hardness. How realistic, or at least "consistent with our current understanding of science," do you want your work to be? You don't need to have perfect consistency; Battle: Los Angeles handwaves most of the tech but has very authentic military operations, with the invaders utilizing the same sorts of strategies and tactics that humans would in the same situation. Conversely you have Independence Day, which was frankly ridiculous on all levels but made a lot more money.
And there are questions of tone to consider. In Real Life, any alien species that found Earth and decided to mount an offensive operation would probably possess technology indistinguishable from magic, and be able to conduct a planet-wide Curb-Stomp Battle with ease. So how realistic do you want to be? True, there might be an Achilles' Heel somewhere in the alien army (say, a thermal exhaust port two meters wide), but how likely is it that the aliens have overlooked it all this time, or that their previous victims never found it? You may need to do some serious juggling to make this work.
Finally, there's the question of whether you need aliens at all. The Ur-Example of the Alien Invasion story, and one of the precursors of Science Fiction as a whole, is The Battle of Dorking, in which Great Britain is attacked and occupied by technologically-advanced, horrifying Starfish Aliens known as... Germans.
The Mohs Scale is a big one here. According to our current understanding of science, Faster-Than-Light Travel is impossible; there are laws of physics preventing it from occurring. So either you're going to break the laws of physics or you're going to have to put a lot of planning and preparation into the alien army. Remember how much work it was for Earthers to put a man on the Moon? Well, imagine putting an invading army on a planet which is about a gazillion times further away from us than the Moon is. The monetary expenditure alone would be, well, astronomical. And you'd need to do your absolute damnedest to make sure 1) the soldiers survive the trip, and 2) the soldiers survive the fighting on the other side, at least long enough to accomplish their mission. Imagine spending $19 trillion on an interstellar mission only to find that the planet's atmosphere is made of magically-anti-Homo-sapiens chemicals and that all the soldiers simply melt upon opening the hatch. Well, as the mission planner (or rather, The Author), it's your job to imagine those things, and make sure your soldiers don't fall victim to them.
You also need to make sure the whole invasion is for logically plausible reasons. To repeat, an invasion is the military outcome of political or economic desires: "Over there, they have some sort of Unobtainium, and we want it, and the best way to get it is to take control of the region by swordpoint." An entire period of world history, Colonialism, was driven by this desire, meaning that a lot of modern consumers—or, at least, those who have taken History classes—know something about the hows and whys of what your aliens are trying to do. Make sure their tactics pass muster to those with a high-school education.
One big subversion is to turn the tables emotionally. One of the advantages of an Alien Invasion story is that the defenders—human beings—almost automatically have the audience on their side, because they (the audience) should be able to see themselves in the defenders. But why are the invaders invading? And what happens if they lose? Tales of Phantasia, essentially a fantasy version of an Alien Invasion story, pulled a massive Plot Twist by revealing that the Big Bad, Dhaos, had invaded to secure precious resources (mana specifically), without which his world would wither and die. Though he became a Well-Intentioned Extremist along the way (the humans of Phantasia had begun too wreak ecological havoc with Magitek, and he decided to wipe them out), the heroes acknowledged that defeating him didn't necessarily make them good people, and that they were condemning the citizens of this other world to death by saving their own. (Fortunately, a Deus ex Machina helped elevate the emotional tone of the ending.) Most Alien Invasion stories are very clear about the consequences if we humans lose, but what are the consequences if we win?
This leads into the next potential subversion: do military conflicts always have to be solved militarily? Humans have always hoped that it would be possible to coincide peacefully with sentient creatures from other planets, and if these aliens are here for something on Earth, they must have something similar to us. Maybe peace is possible, if people with calm heads (from both sides of the war) manage to get in contact with each other. Timothy Zahn's The Conquerors Trilogy revolves largely around this, albeit in a Space Opera background. You have to be desperate to launch an unprovoked invasion of someone else's homeland, as opposed to the minor military skirmish which starts off the war in Conquerors Trilogy. Of course, this desperation may prove useful when it comes time to hit the negotiating table. Consider taking a brief look at the Orson Scott Card work Speaker for the Dead, sequel to Ender's Game, which doesn't focus a lot on his gay politics and instead talks about Inscrutable Aliens and the line between them and, well, non-inscrutable aliens.
Yet another subvertion, somewhat stemming from the previous two, is changing the POV of the story. Vast majority of those tales focuses on humans struggling against faceless, inscrutable alien invader. But how about turning the tables and telling the tale of aliens invading yet another planet to add to their empire, only to run into rows of unexpected problems? An alien foot soldier who starts sympathizing with the enemy? Telling the entire story by eyes of alien - especially the one with morality, psychology and/or culture much different than ours - would allow you to refresh a somewhat cliched genre.
Suggested Themes and Aesops
There are plenty of Green Aesops to be had here. If Planet Looters are taking the effort to go ga-trillions of miles out of their way and invade our little blue planet, they've probably already wrought ecological hell on their own homeworld. There's definitely an anvil to be dropped there.
Somewhat related to this, you can say that Humans Are Bastards, if not the real monsters in this tale. You can show that for all that we demonize the alien invaders, they're not acting that much different from the conquistadores of South and Latin America, or - if you feel like launching yourself into a really deep political mess - the U.S. in Vietnam. When push comes to shove, we're no different, and the increasingly brutal actions of your characters against the aliens can underscore it.
On the opposite end of the scale, you have what 4chan refers to (slightly mockingly) as Humanity, Fuck Yeah - long story short, humans are awesome. We are special, we're warriors, and most of all, when external threat hits us, we band together and kick the hell out of external threats. With the current trend of darker stories and Deconstructor Fleets, this can be a very uplifting aesop.
Insect motifs are rather overused in alien stories. Nevertheless, there's a reason they prevail - they scare the audience. Classic elements of those are of course aliens looking like Big Creepy-Crawlies, the Hive Queen who doubles as the keystone of aliens' Keystone Army and Hive Caste System, with soldier-aliens and occasionally glimpsed worker units.
There are many rarer motifs your alien invader could "wear" - demonic appearance, scary masks hinting at Proud Warrior Race Culture, avian aliens, reptilian aliens, perhaps even Space Elves. Given how recently most alien invasion stories has been aiming at something between mammals and insectoids, mostly everything would be a breath of fresh air here.
Aliens can be monolithic, cold entity whose every warrior is identical and which acts according to rules of logic and reasons. If you chose this, go for futuristic, technological feeling and you'll get something along the lines of Cybernetics Eat Your Soul. On the other hand, a horde of aliens fighting with techno-swords or better, hand-to-hand-combat, will evoke the fear of primitive barbarism, thus leaning closer towards The Horde.
From human side, you can go for "humans are awesome", "humans are struggling" or "humans are helpless", which fit in at the most recognizable points of Sliding Scale of Cynicism Versus Idealism. Independence Day is the "awesome" and most idealistic variety. At first, aliens hurt us badly, but when we get our bearings, we proceed to hurt them worse with little actual problems. "Struggling" version, however, sits squarely in the middle of the scale and while humans are capable of fighting back, in this world you need to Earn Your Happy Ending. "Helpless", on the other hand, lends itself for more depressing stories, even though technically it's probably the most realistic realistic. Aliens are more advanced and the best humans can do against them is to form La Résistance and hope that opportunity to destroy the invaders will present itself.
The most basic outline is, of course, that aliens invade the Earth and mankind must defend itself. However, there's a lot of flexibility here. Will your story follow a global defense, jumping from POV to POV in attempt to chronicle a world-wide war? Will you present your readers with only a small piece of a greater tale, showing soldiers warring against the invaders in just one country/region? Or perhaps you're going to show struggles of small, isolated base which has lost contact with the rest of the world and has no idea if anyone but them is fighting anymore? You can also eschew the military completely in favor of showing a group of civillians who try to survive and find sense in the world that has just gone mad.
Another possibility is to set your story after the invasion. Have mankind won, then, or have they lost? If we lost, then the main focus of the story is usually on La Résistance, but why don't make your hero The Quisling who aids the invader (now occupier) out of fear, chance at personal gain, or simply because he believes that aliens are better managers than people? If we won, on the other hand, then perhaps we need to find a way to live with survivors of alien army, or we're hunting those survivors down, or maybe we're setting into space to deliver inverted alien invasion to our attackers?
The invasion doesn't have to be global. Perhaps your "invaders" are a bunch of really homicidal aliens which have crashed their ship right outside of some god-forgotten town and decide to raid it to kill everyone. In this case, you should take a good look at Horror Tropes.
A rarely seen option is to focus on diplomats who try to negotiate peace with aliens. This requires a bit more trickery, as there needs to be a reason why species that's attacked as first would now want to negotiate with us. Perhaps there may be dissent among aliens, or even a full-blown coup d'etat.
Something that's pretty much absent as the main plot is humans trying to acquire alien Applied Phlebotinum. Sure, this happens, but mostly as a sub-plot. How about a story of a spy invading a ship full of Human Aliens to find blueprints for their Wave Motion Guns or to blow up their ship while bombs fall on Earth beneath their feet?
And of course, as already mentioned under Subversions, you can write the entire story from alien perspective, shedding a new light on both mankind and invaders themselves.
Set Designer / Location Scout
Just about anywhere on Earth, when you get down to it. Hell, one crazy filmmaker had his aliens popping out of the deepest part of the ocean!
A bit more to the point, the choice of your location will dictate the tone and atmosphere of your film. Setting it in the city is unsettling, because with all the tall structures blocking our line of sight and sound acting oddly in blocks-created canyons you can never know what awaits you behind the next corner. Ditto for woods and caves, with added bonus that in twentieth century, those are alien surroundings for many of us. On the other hand, setting your story among fields, on great expanse of grass, lends itself nicely to feeling of loneliness, but also means that your characters need to keep constant vigil - if they can see aliens, aliens can see them, too. Yet another option is to put your story on a ship, or in a maze of abandoned skyscraper. In addition to things mentioned about cities and woods, this lets you craft atmosphere of isolation and mounting paranoia, especially if contact with outside is lost.
Go for Scenery Gorn - nothing underscores the threat and evilness of aliens like showing cities blown to smithreens or villages slaughtered and burned in the wake of invaders' raiding party. Some gratuitous Monumental Damage is fine and dandy, but don't overdo it too much - headless Miss Liberty and falling Golden Gate bridge are so cliche many of us would probably be surprised to find the two of them intact in Real Life.
Does your story take place mostly in the light of the day or by night? Day lets you show the danger clearly and is better in more idealistic and optimistic tales. On the other hand, night underscores danger, uncertainity and isolation, thus giving you more narrative tension. Then again, you can make readers bite their nails even when sun is shining bright.
This is where you can let your imagination run wild. What do the aliens use? Do they fly in a classic Flying Saucer, or a more menacing triangle? Do they use ray guns, or do they still have projectile weapons? Do they require space suits in our atmosphere, or not? Also, be sure to have the tech be consistent. It may not make too much sense for aliens whose technology is mostly organic to have huge metallic super computers.
Remember, also, to fit the weapons and spaceships to your aliens. If the creatures are three- or four-handed, have their heavy guns need all hands to work. If your aliens are quadrupedal, don't have their spaceships' cockpits be fitted with human-style armchairs. If they're blind, they don't need translucent windows, or any windows, for that matter. Think what would make sense from their perspective, not ours.
As for humans, go for real-life military technology. Make fighters engage spaceships, match heavy sniper guns against aliens' futuristic guns. Apart from that, however, remember about weaponry not often seen in alien invasion movies. Imagine tanks engaging large alien structures and keep nuclear submarines in mind.
For aliens: any armor or spacesuit they might require. If you go for more savage type, make them wear no armor and have unnaturally strong natural hides, or perhaps armor that looks like it's part of them. The higher on apparent tech level ladder the aliens climb, the more sci-fi-ey the armor will look. The ones in the middle of the scale might have bulky and heavy Power Armor. The highest ones might go for something that's close to their skin, yet provides protection. Go for sharp edges and flat surfaces rarther than curved, spherical shapes which make it look as if child safety specialist or Apple salesman has just finished inspecting the invasion fleet.
As to aliens' masks, today most go for Faceless Goons approach, where all enemies look identical. You can take this in many different ways - perhaps they're The Blanks, or maybe there's some humanlike, yet foreboding or Uncanny Valleyish quality to them. On the other hand, if you're feeling creative, make each mask different. Why? Perhaps it denotes the clan this particular warrior comes from, or calls for favor to aliens' chosen god, or maybe aliens are painting their masks for quick identification on the battlefield, or just to stand out. Going beyond Faceless Goons can flesh out your aliens' culture. On the other hand, if your aliens have terrifying faces by themselves, you can throw helmets away completely and scare your humans with mandibles or More Teeth than the Osmond Family.
For humans: whatever fits with the time period, really. Camos and military grab for soldiers, civilian clothing for non-soldiers, lab coats for scientists. Keep it reasonable - running from alien threats in miniskirt and high heels might be Fanservice incarnate, but not only does it look awkward, it's likely to end up with Ms. Fanservice falling over, or freezing, or getting hurt in unprotected parts of her body. It's like they tell you in real life - dress sensibly and appropriately to occasion.
The Everyman is often a good protagonist, just someone who ends up getting caught up in the global devastations. Your usual reader or viewer has the highest chance to sympathise with character like this and see himself in him. If you decide on this archetype, he's unlikely to be a badass who takes aliens down with rocket launchers and hand-to-hand comabt - more along the lines of Action Survivor. This also limits the scope of your work, but if you don't want to focus on military operations and global scale, that's the right man for you.
Military types get to enjoy more action in those types of settings. They're our first line of defense against any invader, after all. They're unlikely to be lone wolves, too - The Squad is necessary. They will see the most action, they will know the high commands' plans... and they'll quite probably be surrounded by scores of Red Shirts dying by droves. The thing to consider is also that their training makes them unnaturally calm when faced with combat or devastation, which may make it hard for reader to identify with them. Nevertheless, if you want to take fight to the enemy in professional manner, this is your guy.
Science Hero is usually a member of supporting cast rather than the main character. Him and his lab-mates are working on cracking the secrets of Bizarre Alien Biology and how to turn it against the invaders. He's the one to sprout Techno Babble and provide the story with Applied Phlebotinum or more down-to-eath method of taking down the invaders. He's also the one to function as Mr. Exposition, speculating on alien origins, setting down the rules of the 'verse for the readers and provide Mission Control. If you want to focus on "why" rather than "how" or you prefer more though-out approach to fending hordes of aliens, this character is for you.
Finally, don't forget Character Development. Any character can start out as one of these archetypes; that doesn't mean they have to end the story that way. One of the (few) storytelling beats the Michael Bay Transformers Film Series did a good job on was chronicling the gradual evolution of Sam Witwicky (Shia LaBeouf) from helpless, have-to-run-away-from-everything (I-mean-holy-shit-they're-100-foot-tall-robots) Action Survivor in the first film to, in the third, Badass Normal who can David Versus Goliath a Decepticon and win.
As for the aliens, this has been discussed in depth in above sections, especially Choices and Motifs. For additional clues, you may want to take a look at Create Believable Aliens and Design an Alien Mind. Whatever you settle for, just remember that your aliens come from very martial, very genocidal, very full-of-themselves or very desperate culture, otherwise your plot just plain wouldn't happen.
Scenes of global destruction are a staple of alien invasion stories. Cities are ravaged and destroyed to any point between smoking, holey skyscrapers and large crater where a sprawling metropolises used to be. Monumental Damage abounds. Planes and fighters fall out of the sky like flies and people and places are strafed.
Closer to the ground, you can show your aliens mowing down soldiers, civilians or both. In fight, humans will likely make use of as much cover as is available, while aliens either No Sell them and just go through the bullets or use cover themselves. In hand-to-hand, it's nice to show humans as somewhat inferior in terms of strength. Show aliens smacking them around a few times like it's nothing. Have humans be the Guile Hero to aliens' The Brute.
Out of combat, have your characters play deadly hide-and-seek with aliens, trying to sneak by the enemy they have no chance to match. Those are scenes that work the best at night and if done correctly, they can be more nerve-wrecking and memorable than a dozen shootouts.
Oh, and remember to include lots of Stuff Blowing Up. People love that.
As someone once said, it's always best to start at the beginning. H.G. Wells' The War of the Worlds is the founding work of the genre. Its two main film adaptations, The War of the Worlds (1953) and War of the Worlds (2005), both explore different aspects of the invasion, the former focusing on the broad global invasion and the latter on a single family's struggle to survive.
For an invasion based on subterfuge, Jack Finney's The Body Snatchers is a great work which explores paranoia and loss of identity, as do its 1956 and 1978 film adaptations.
The Epic Fails
Make sure that the aliens invade for logical reasons. One of the recurring criticism’s of Signs is that the aliens are allergic to water, and yet they invade a planet mostly covered in water with high amounts of precipitation while walking around completely unarmed. Also, try to ensure that your characters are likable. The latter two adaptations of Body Snatchers make the characters so unlikable that you may cheer for the pod people to win.
For another variety of a fail, a good lesson on importance of Foreshadowing and keeping to your rules in story is Out of the Dark. While a So Okay, It's Average book overally, a combination of Unexpected Genre Change and really good Red Herring resulted in a Twist Ending Deus ex Machina of epic proportions. Read it, if only to see what went wrong.