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Fix a bedtime and a getting up time. Don't allow these times to drift. The body gets accustomed to falling asleep at a certain time. Even if you're not working, this is an essential component of good sleeping habits. Avoid sleeping during the day. You won't be able to sleep that night. Avoid caffeine . This includes drinks like Coke and Red Bull. Use comfortable bedding.
Exercise regularly, but not shortly before going to bed. Regular exercise, particularly in the afternoon, can help deepen sleep.
Find a comfortable temperature for sleeping and keep your bedroom well ventilated. If your bedroom is too cold or too hot, it can keep you awake. A cool bedroom with warm bedding is usually the best combination. Block out all distracting noise, and eliminate as much light as possible.
Don't use the bedroom as an office or a lounge. Associate the bedroom with sleeping. Or sex.
Don't take your worries to bed. Leave your them behind when you go to bed. If you're awake at 4am fretting about something, it'll almost always grow until your worry becomes overwhelming and insoluble. Learn to ignore your night-time worries - tell yourself you'll worry about them at 10 am instead, when they'll be much more manageable.
If you don't fall asleep within 30 minutes, get up, go into another room, and read until you're sleepy.
Most people wake up one or two times a night for various reasons. If you find that you get up in the middle of night and cannot get back to sleep within 15-20 minutes, then do not remain in the bed trying to sleep. Get out of bed. Leave the bedroom. Read, watch telly, or have a bath and then try again.
Many people fall asleep with the television on in their room. Watching television before bedtime is often a bad idea. Television is a very engaging medium that tends to keep people up. Some people find that the radio helps them go to sleep. Since radio is a less engaging medium than TV, this is probably a good idea.
Further reading (external sites)
broadly based on The University of Maryland Medical Center's page.