When I was a kid, these were the best thing about going to the movie - the first fifteen minutes that gave you a sneak peak at things to come. With the internet and technology, things have gotten slicker. Trailers are a given on every DVD, and you can download them on the internet.
Every movie has a web site.
You can find them all here. I don't really know what makes a good trailer. Just like I'm not sure what makes a good movie. Important sense of mystery. Yeah, you know who he is now.