I've actually found that forced "tangential" thinking makes me fall asleep really fast. Basically, free association but you have to move onto the next thing instantly (eg. Think of a feather and then it's like a comb and maybe the comb is green then green grass, etc). I just seamlessly transition into dreaming. The key is not dwelling but moving on right away so you don't get stuck on things like "where are my car keys" or whatever.