Caffeine is a well-known cause of sleeplessness, found in cola, tea, cocoa, as well as in coffee. Nicotine can cause shallow sleeping and sleeplessness. Have a healthful bedtime snack. Try a small glass of milk and a piece of fruit. Experiments show that people sleep better if they aren't hungry.
Some people work too hard at getting to sleep.
If sleep doesn't come, read a book in bed or listen to music until you feel tired.