There are many different things you could try.
Firstly though, do you go to bed and watch telly, read etc while in bed? If you do stop. Your subconcious will associate your bed with this mental stimulation. Make you bedroom for sleeping only.
Set yourself a routine to prepare yourself to sleep, shower etc. Avoid stimulants before going to bed, tea, coffee, smoking etc.
As mentioned, exercise. Everyday. Not just a relax stroll around the block either.
Hows your diet? Avoid high sugar/ saturated fats laden foods. Also avoid things laced with preservatives. Lots of fruits, veg and grain. Low GI toward the end of the day.
Have a banana shortly before going to bed. Potassium helps with sleep.
If you're having trouble sleeping get up, go the the lounge and read for a bit and try again. Don't get frustrated. It will heighten your awareness/mood etc and make relaxation difficult.
Try meditation.