This sounds a bit nuts, but it works for me (sometimes).
Get into bed, try to relax as much as possible and settle down as if you were trying to go to sleep...but, keep your eyes open and make yourself keep on keeping them open for as long as you can.
Only other thing I would say is, try not to lie in if you've had difficulty in getting off the night before. I know it's hard, but it will only make getting off the next night harder.
If you haven't been sleeping well for some time, you could approach your GP or Practice Nurse for advice. Some places have sleep clinics.