What makes a successful trailer?

There are many ways in which to make a film trailer successful. These ways include;

Not giving too much away

Don't be too cryptic 

Speak to the audience to attempt to aim your film at 

Introduce some, but not all of the characters that will feature in the film 

Capturing the right mood in keeping with the genre of the film 

The sound must be interesting. For example; Voice-overs, no sound at all etc. 

The editing used such as; Blurs, quick cuts etc