If my death will trigger whatever nonsensical force causes my evil lairto collapse, I will make sure this also applies with all imperial public structures such as aqueducts and mine shafts (just like my lair, I built most of them anyway). And I will remind the heroes that killing me will mean years of drought and mineral shortage to millions of people.
Better yet, in addition to the above, I will have a doomsday device that detonates upon my death. This will make it far more difficult to defeat me.
If I manage to capture one of the heroes, I'll figure out what kind of weakness allowed me to capture this one at first. Then perform other methods like psychic interrogation, DNA scanning and computer simulations for further discovery on their weaknesses and abilities. This data will then be present at the next advisers meeting for analysis and exploiting it.
I will have minions search out every weapon merchant in the land and either employ him or buy out all his stocks. With any luck the heroes will have no access to equipment above the starters.
If I have other means to gain control of the kingdom than to force a princess to marry me, I'll go with this plan instead. Also, I'll make sure to remember that "princess" is not synonym to "first beauty in the country" or to "perfect wife for evil lord."
A backstabbing wife can be dangerous to have around, but she can provide necessary training. If I never relax in her company, I'm less likely to relax when the hero comes to assassinate me.
I will pretend to care for my minions to keep morale up, but in reality I will be sure that I do not, in any way, care about them. That way, I won't go crazy should they turn against me.
The architect who designed my dungeon, its traps, and its escape routes is doomed from the moment he signed on. As are any external service personnel I absolutely must hire to provide upgrades, such as the poor sod who put in the central A/C.
I will regularly train for combat with my most loyal and skilled henchmen. It's always important to sharpen my abilities, especially if I haven't gotten much personal experience in fighting.
I will plan according to my budget. If I do not have the money to design and mass produce the kind of technology required for many of the above precautions or if I don't have the good managers required to supervise my minions and make sure they actually follow protocol at all times, I will try to find a less expensive option and fix what I can.
Any idea which crosses my mind which seems to only be there because I think it would be cool, regardless of practicality or good sense will be ignored. Yes, developing bands of minions to act to popular character types would be pretty neat, but I wouldn't want to be distracted from my overall plans by the little things.
If a minion or civilian suggests something that is halfway decent, I will give him the credit for a good idea, rather than just stealing it. That way, if it fails, I can finger-point.
If for whatever reason, I need to talk with the hero while holding him at gunpoint, I will use three highly trained marksmen. One to shoot at him from the front where he expects it, one to shoot him after he dodges the first bullet, and one with a machine gun to completely obliterate the corpse.
All weaponry that can overheat and explode in my face will have a temperature-operated failsafe, that will render the weapon completely inoperable if removed or tampered with, except for repairs and upgrades.
My doomsday devices will not have a lengthy charge-up time, and will have a countdown timer just for show. They will be ready to use as soon as the hero waltzes into my control room. In addition, the conspicuous "off" button will activate the device immediately.
If I take up playing an instrument, I will stay away from the ominous pipe organ. Marbe I should pick up something less obviously evil, like a trombone. Oh Wait.
I will frequently pretend stupidity. If the hero is genre savvy, he or she will be afraid and think twice before attacking. If not, I get an easy kill.
I will bring recruits into my army by funding their education at my nation's top school, thereby ensuring they are intelligent and capable of quick thinking.
I will always value a small force of capable people (read: able to cause more than a few minutes' setback to the hero) over a vast army consisting largely of weaker mooks.
I will never wait with renewing whatever it is that sustains my life, but will do it as early and as often as possible.
If I must sacrifice a virgin every full moon, then there will be a harem of virgins kept well fed and treated but always prepared in different locations. Holding off the search until the last minute will just make me do something really stupid, like go after the hero's girlfriend merely out of desperation.
If my mad scientist creates a bunch of fully sapient monsters, with the emotional capacity for holding grudges, I will make sure that they aren't discriminated against. It would be disastrous if they betrayed me because I let them be treated as sub-human.
Any secret agents or otherwise heavily trained soldiers from a distant land will be considered threats to my power.
Before I turn myself to the dark side, I'll find out if it does in fact have the overwhelming power that it promises. If it turns out that the greater power will leave me at a huge disadvantage to the forces of good, then I will choose instead some other means to make my reign of evil a reality.
I will always remember the motto: "Overkill is good, overconfidence is not." If I kill a hero, I will not relax until I detonate his body to pieces, incinerate the pieces, fling the ashes into outer space (or better, into another dimension), and exorcise the site.
Loud alarms in my base are reserved only for when an approaching army is sighted. If a lone enemy is sighted, a silent alarm will be raised and guards will simply patrol (but not appear to notice) the intruder, instead using the patrols to force the enemy into the dungeon, where they will be met with by my Elite Guard.
If I build a secret room into a house, it will be underground, not behind a secret wall where anyone with a blue print or a decent sense of dimensions and spatial relations can spot it.
I will also make sure the plucky comic relief cannot find the secret room by stumbling around and tripping into it.
My security cameras will be placed in such a way that the hero cannot be out of sight by standing directly under the camera mounting.
Just in case a hero has someone to take the bullet for them, I will always have an extra bullet more than I need to execute heroes. Furthermore, I will capitalize on the time it takes the hero to hear the rescuer's last words to line up my next shot and kill him.
If the end of my rule is inevitable, I'll search for a way to fake my death, establish a cult promising my messianic return, and preserve my body until the world is once again ready for me to take over.
I will order all copies of the Evil Overlord List, and all subsequent derivatives, to be destroyed with the exception of a master copy which will remain in my safe deposit box for reference.
My soldiers will wear devices that will sound an alarm if they are completely motionless or fall flat.
If possible and necessary, I will turn myself into a powerful and free-willed undead who can resurrect after being killed again, such as a lich. This way, I'll just keep coming back. Of course, my soul jar will be under heavy guard in a secret room, with doors that only be opened from inside.
Should I decide to split my soul, I will NOT keep the parts in unique and gloriousitems. Instead, I will use grains of sand in the Sahara and the Atlantic Ocean, and a small chunk of ice in the Arctic. And a screw on one of NASA's deep space probes.
My soldiers will immediately destroy any towers and church steeples they encounter, as these are great sniper-blinds.
I will not torture or kill the hero's friends in front of him. This increases the chance that his hidden powers will activate and kill me. I will take special care not to provoke the hero after I have won the fight.
I will have a large group of highly-trained snipers guarding all sides of my fortress from any conceivable angle. One third of them will have laser sights. The rest will have scopes.
All powered armor, mecha walkers, and battleships (both of the seafaring and spacegoing variety) will have point-defenses, even if such measures are largely worthless. Better safe than sorry.
Rule #4 will not only be my personal guideline, but also an official policy for all soldiers. There will also be a policy to always shoot a body twice.
Rather than use an obvious evil fortress, I will base my activities out of a reasonably-sized office suite that hosts a variety of offices for more legitimate businesses. The heroes should be hesitant to attack when there are so many innocent bystanders.
I will not employ huge three-headed dogs who can be won over with honey to guard my ultimate weapon. Instead, I will employ small and fluffy squirrels, who just so happen to be vicious carnivores, and will massacre the hero before he realizes his folly.
I will never tell any of my henchmen that "I don't pay you to think." Yes, it's true that people who think too much might be a threat to my authority, but if I wanted a mindless robot, I'd make or hire a mindless robot.
I will fool the heroes by building a false headquarters, misleading them into thinking that this is my personal base of operations. Once they enter expecting a climactic showdown, they will instead be locked inside with explosive booby-traps to level the entire building.
Should I ever meet the hero, I will instantly pretend to be my own "good twin", then offer to lead them to my "evil twin's" lair. My soldiers will "kidnap" me when we approach, and thus lead the hero and his party into a trap.
I will ensure that my henchmen and servants get to throw fun parties when they're off duty. Also, if the hero walks in on a group of people playing charades where someone is impersonating a goose, he will think twice about it being his nemesis's fortress.
If a person carrying an oddly-colored sword walks into my kingdom, I will take a vacation elsewhere until that sword is out of my kingdom and not make any friendly or unfriendly overtures to the wielder. If the sword is black, I will move to another dimension and never return.
If I'm ever at the mercy of the Hero, I will share any (real or false) stories about my unhappy childhood. If the Hero is empathetic towards apparently sympathetic villains, he would at least spare my life.
There will be no architectural elements in my bases that can provide alternate pathways for intruders - no catwalks that go nowhere but provide footholds, no entirely pointless machinery to climb, no convenient handholds that serve no obvious purpose besides being convenient handholds.
Even if all my advisers agree that no one could survive that blow I have just dealt to the hero, I will still chop his head off for good measure.
If I'm ever seriously injured, I will fake being knocked stupid.
In a sealed and airtight room, I will have all my minions wearing gas masks stand next to bright red barrels labeled "DANGER: EXPLOSIVE!". When damaged, the barrels will release a non-flammable, non-explosive poisonous gas that will kill anyone not wearing gas masks.
If I create mutant super soldiers, I will not make the prototype my trusted lieutenant, as prototypes have a tendency to go insane. After field tests prove successful, I will kill the original and clone more stable and advanced versions.
Playing ominous music that screams finality, nice as it is, tends to give the heroes the resolve to continue all the way through my citadel and confront me. Instead, when they invade, I will begin playing elevator music, which is much more likely to exhaust and depress the heroes.
When hiring new troops, I will tell them that their stealth ops uniform is neon pink. Anyone who tells me how impractical that is will be promoted immediately.
I will take a fairly neutral stance on transhumanism. It will not be mandatory, nor will non-modified citizens be discriminated against. It will also not be illegal to undergo cyborgization or bio augmentation.
If I foolishly accept the hero's love interests' offer to become my consort, I shall never stop being suspicious of her, no matter how much time passes. Should she ever ask me about my weaknesses, I will lie.
If one of my lieutenants turn out to be a traitor, I will kill him, cut off his head, impale his head on a stake, and nail a sign on said stake that reads: "I do not tolerate double-crossing back-stabbers", for such people tend to be more trouble then they're worth.
I will create a centralized licensing and registration system for all weapon manufacturers. All arms sales will be registered and tracked.
I must remember that in the grand scheme of things, there isn't such a thing as complete immortality; I can be ageless, invulnerable, or both, but there will always be a way to shatter the magic and kill me. I will always keep this in mind no matter how assured my victory is.
If I beat the heroes to any sort of magic artifact that doesn't involve my own plans, I'll destroy the thing on the spot, not throw it into my dungeon in an easily accessed spot, the only exception will be if destroying the artifact will remove my powers.
The corridor leading to my artifact of doom will be full of highly visible and sweeping laser beams, that will distract intruders from the grid of invisible laser beams; as well as the sound, heat, and motion detectors.
I will improve on every other villain's ideas and potential villain's ideas, including but not limited to this list.
If I rise to power by replacing my boss, I will not lash out if my former leader returns, odds are whatever helped him cheat death could turn me into a puddle of molten flesh in an instant. Instead, I'll return to serving him until I find what his weakness is, and then direct the heroes to it so I can take him out when I get the chance.
If I use any sort of long and complex deathtrap, I'll make sure that in case of escape, I will personally implement it. That way, when the hero thinks he's been smart and escaped through the absurdly spacious sewer, or wherever it may lead, I will be waiting. With a gun.
If I absolutely must have brainwashed henchmen, I will not give them the keys to my inner chambers.
Before I send my assassins to kill someone, I will give them bags full of hair trimmings collected from the local barber shop, or other random genetic material, to contaminate the crime scene with.
I will prevent the creation of potential heroes by appointing their families to important positions in my government, and molding any would-be heroes into my right-hand men.
I will remember that the power of hope is not the exclusive virtue of the good guys.