What is a book trailer? Well it's kind of like a movie trailer, only instead of getting people excited for movies, they get people stoked for books. If you go to YouTube and search for "book trailers" you get about 17,000 results. You can narrow your search however. When I changed the search to "YA book trailers" I got 158 results.
Now, not all of the trailers are something I like. I watched one for The White Gates by Bonnie Ramthun (which comes out in November 2008). If you like three minutes of watching snowboarders crash, it's great. For a book trailer, I think it just dragged out a little too long. Then I found another one for the same book that's about a minute long and much more interesting.
I also found a book trailer I love for Wake by Lisa McMann. It's also about a minute long. The only drawback is the ad banner that pops up across the bottom, but the rest of the trailer makes up for it.
And here's one more example of a book trailer. This one is for Airhead by Meg Cabot. It's a little different from the other two.
As you can see, I prefer book trailers that are around a minute or less. It could be that I just have a short attention span, but a lot of the videos seem to run less than a minute and a half. So, if you have a book that needs a trailer, make it interesting. Keep it short. And give it good music if that's the way you want to go. Basically, keep it professional and make it something to pique curiosity in your book. I know, easier said than done, right? The best advice I can give is study the book trailers that are out there for your genre, focusing most of your attention on the videos with the most views, and don't be afraid to try it again if your first attempt falls flat. Remember, I found two videos for The White Gates by Bonnie Ramthun. I would suggest that if you do make more than one trailer because that first just wasn't doing it, take down the one you didn't like. You never know if that's going to be the only trailer a potential reader will see.