The Nemours Foundation offers this advice:

• Make sure your child drinks plenty of fluids during the flight. Water is best, but make sure the drink doesn't have caffeine.
• If the child usually has ear pain associated with plane travel, give the child a pediatric dose of acetaminophen or ibuprofen about a half an hour before takeoff and landing.
• If at least 3 years old, let your child suck on hard candy or chew gum.
• Bring a bottle or pacifier along, or plan to breast-feed during the trip.
• Encourage your child to yawn frequently.
• Have the child use a nasal decongestant spray before takeoff and landing.
• Don't allow napping during takeoff and landing, as it's more difficult to keep the middle ear equalized during sleep.