Some things that can help:
- Exercise during the day. It's a no-brainer but it helps a lot
- Don't use your bed for ANYTHING else than sleeping (well, sex too, I suppose). Using it for other things (reading, watching, TV and such) sends mixed signals to your brain whenever you lie down. If you can avoid it, in fact, don't use your bedroom for anything else than sleeping.
- If stress keeps you awake, try to find a way to deal with your problems so you don't take them to bed with you
- Try to build in a fixed rhythm, and always go to sleep at the same hour if you can manage
- Don't take sleeping pills. They build dependance, and a drug-induced sleep has minimal recovery value
- If you need to wake up early, be sure to go to sleep in time. Frequent awakenings are often caused by the stress of having "only five more hours" of sleep.