My aunt is a professor of kinesiology and specializes in sports injuries and rehabilitation. This Christmas I asked her this very question and she gave me some advice that worked :)
Lie still. Tense you toes. Count to 5. Relax them. Tense your feet. Count to 5. Relax them. Continue this up your body (calves, thighs, buttocks, abs, back, chest, shoulders, upper arms, lower arms, hands). After this you're going to feel pretty relaxed. Focus on nothing but relaxing. Relax your feet, ankles, legs, etc all the way up your body (not flexing this time).
Once your whole body is relaxed you'll be well on your way to sleepyland.
Or, you know...a few shots of whiskey in some hot chocolate does the trick.. :)
OR.. A shot of rum and some NyQuil. O-u-t. lol