I have no idea how to fall asleep. I agree with Jeremiah, if you don't fall asleep then get up and do something for an hour or so.

As to what I3dx said, I'm pretty much always thinking about something and that's probably where most of my insomnia comes from. I don't know very many ways to turn it off, but I can tell you that thinking about a problem will definitely keep you from sleeping. I guess it's kinda like trying to sleep while running a marathon.