Make the bedroom a sleep-only zone. Remove most toys, games, televisions, computers, and radios if your child is having trouble falling asleep or is frequently up at night. These items can be powerful cues for wakefulness. This goes along with the recommendation above of limiting time in bed. One or two stuffed animals are acceptable.
Sleep problems can be very frustrating.
There is an excellent book written for lay persons on this subject by a colleague of mine, Dr. Richard Ferber, at Children's Hospital. It is called Solve Your Child's Sleep Problem . It does a far better job than I could do myself in treating this important topic.