Drinking a small glass of warm milk has helped me on several occasions.
Also, whenever I lay down to sleep, I make up situations in my mind, and I almost become dream like (therefore tired). I mean make up situations such as "what would happen if I was involved in a bank robbery (what would i do, not do, etc)" or "If everyone on Earth except for a few people died on Earth, what would i do to survive, etc (this usually involves stealing a Ferrari and going real fast down a highway
).
This is a good way to keep the mind active while making the body tired. Also a good start in learning how to lucid dream...