Trailers are short ways of advertising a work. They need to make the work look attractive, interesting, and most importantly, they need to look like they're advertising what it is they're advertising.
- The Main Characters. They can be shown with their most character establishing quotes, or their most dramatic moments. There ought to be a lot to your character, so cut it short. Show them, show their motives.
- A MacGuffin. The object everyone craves. Even if it's a living MacGuffin, this goes best with someone else describing why it's so important over as we see the object, or even as we see nothing but it's shadow, or its case. Keep it mysterious.
- A monster. If this a horror or an action film staring some sort of monster, literally or otherwise, then it's essential that Nothing Is Scarier is in play. Show the monster for a moment, at the beginning or the end, but make it focus around the aftermath of what this thing has done. Make them question what is it that makes this thing horrible, but don't show the audience where it came from, how it works, what it truly is... just show that the characters (most of them anyway) are genuinely terrified of this thing.
- The Bad Guy. There's a lot of ways the Bad Guy can be shown in film, but the trailer shouldn't pick much more than one evil trope, and be done. Like with the monster, the bad guy should remain reasonably unknown, but he or she can be shown in a different way. Tropes such as Evil Is Sexy, Evil Sounds Deep, Faux Affably Evil, Face of an Angel, Mind of a Demon, and Game Face are all ways you can show off your bad guy and show that they're more than just another character without showing too much away.