Are you a reader? For me, reading a book in bed is a sure way to drop off before I can get ten pages in. (If I want to read more than that, I have to sit up.)
If reading doesn't put you to sleep, try some quiet music or even the TV on, turned down low. Complete silence can be hard to fall asleep in if you're having trouble sleeping, but a little distraction can help sometimes.
And if that doesn't work either, do something else that requires your mental attention, but nothing very physical. Don't do exercise to make yourself tired (as some people might recommend). The adrenaline will keep you up for hours more. A person doesn't sleep because their body is tired, but because their mind is tired. Tire out your mind and you'll fade away into dreamland.
I'm a psychosomatic sister running around without a leash.