The art of the really good teaser trailer seems to have been lost, or at least ignored. Done properly, a trailer reveals almost nothing about the actual plot while reeling you in to go see it. The trailer for the original Alien was a masterpiece of this. Not a single word of dialogue, not a single spoiler, but man alive does it make you take notice!
http://www.youtube.com/watch?v=bEVY_lonKf4
These trailers for The Shining and Magic (must be a horror movie thing, LOL) were also very effective:
http://www.youtube.com/watch?v=Lx7smh7YHKg
http://www.youtube.com/watch?v=piQFD4gz9l8