I often read a really boring book OR I start looking through my crochet and knitting patterns (which aren't for boring projects at all). My mind starts trying to figure things out and all of a sudden I'm sleepy.
I used to have this great college book from my historical geology class. I could read about a page of it, toss it over the side of the bed, and be out like a light.
Are you exercising too late or have you messed up your schedule lately? Both can cause insomnia!