Building an awesome trailer can be troublesome. From the get go I've build my game to have surprise after a surprise and you shouldn't spoil stuff in the trailers like they did with the movies of olde. I could show quick flashes of stuff that's about to happen, but all the great stuff is based on more or less somehow being surprising. What to do?! Luckily I've found this handy guide:
How to make a trailer, 7 easy steps.
Maybe it'll help some.