Impressive trailers for your game can really help show players how awesome your game is, but I'm having trouble finding tips that describe how to create a great trailer. I would like to learn about the best way to decide trailer length, structure, music/sound, and content. Basically, what parts of my game do I show in a trailer, how should the scenes be organized, and how would you do the sound?

Any advice on things to avoid would be great as well; for example, I read that splicing some video, switching to a screen with text, switching back to video, etc., probably isn't the best way to do it. Thanks!