I actually have a pretty good trip that worked when I went away to camp. This trick is really easy, but takes a lot of self control.
Lay in a very comfortable position, close your eyes, and take a deep breath. Imagine your toes falling asleep. They should feel different, and you’ll want to move them, but you can’t move a finger during the whole thing. Take another breath and imagine your feet falling asleep up to your ankles. Repeat, imagining your body falling asleep bit by bit, but DON’T MOVE AT ALL. Seriously, you’ll have to start all over. Usually, by the time I reach my shoulders, I’m asleep.
I don’t know if it’s a form of hypnosis or something, I’m pretty sure it’s just the fact that your focused on something other that what’s around you. Another thing you can do is count to 100 in your mind, breathing deeply. When you reach 100, count backwards down to one, doing the same thing. You may have to do it a few times, but when I use it, I’m usually asleep halfway through the second time. Hope this helped.