Hook - The hook explores, without giving the ending away, one of the most exciting or interesting thing that happens in the book.
Audio - Music has to match the mood of the book.
Keep it simple - Less can be more when telling a quick story about a story.
Endorsements - Endorsements and reviews should be simple and the trailer should not over rely on them for building interest (publishers really struggle with this one).
Audience - Who is the reader the trailer is trying to reach? Are they successful?
Don't give away the ending! - It is truly a bummer for all if the ending is given away.
