One of the most important things (or the most important thing) for a movie is the trailer. Trailer's make the viewer decide instantly whether they will see the movie or not. It is supposed to give you a glimpse into the movie's purpose and style, but not give away way too much. Here are some examples of BAD movie trailers:
1. The Backup Plan -- You can see the entire plot just from the trailer.
2. Alpha and Omega -- Ditto.
3. Millions of other movies
Here are some examples of GOOD movie trailers:
1. Cloverfield -- Mysterious, engaging, terrifying.
2. Catfish -- Tells you absolutely nothing, but you want to know everything.
3. Where the Wild Things Are -- Used music very well and only gave a glimpse into artistic look of the film.
Gotta check it out chick-a-dees. ;)