A sleeping child is a happy child. Sometimes the best thing you can do to make your kid on a flight is to schedule it for his or her sleep time. This may mean a "red eye" flight or one that takes place during regular nap time.
If your child does fall asleep, you should do the same, if possible. A few minutes' nap can help you recharge your batteries so you can stay focused on your child when he or she wakes up and wants something to do. If you can't sleep, close your eyes, let your mind go blank and relax.