To help your baby's ears adjust to changes in cabin pressure, encourage swallowing during takeoff and landing. You can do this by breastfeeding, or offering a bottle or pacifier. Toddlers can take a drink, nibble on crackers, or suck on a lollipop. (Look for those without a gum or chewy center, which can present a choking hazard.) Use the feeling in your own ears to determine when to give your baby something to swallow, or feed your baby when you see the flight attendants preparing the cabin for takeoff or landing. If your baby is sleeping soundly, don't feel you need to awaken him; he'll be fine.
Flying in an airplane can cause dehydration, which occurs much more quickly in a child than with an adult. Keep your baby well hydrated with water, juice, or milk.
Changing diapers can be a real challenge. Some airplanes have changing tables, but these are typically very small, and while great for newborns a tricky challenge for bigger babies. You can ask the flight attendant for the best place for changing. A small baby can be changed on your lap on or the pull-down tray table. (Be sensitive to the people seated near you if you do this.) Some airlines will allow you to use the flight attendant's jump-seat; some will let you change your baby on the floor near the galley or in the bulkhead area. If you have an older baby, consider using pull-up disposable diapers on the flight, as these can be pulled up with your little one standing. Use a plastic bag from home or the airsickness bag for disposal in the bathroom trash. Remember that, since flight attendants handle food, they can't handle dirty diapers. (And they probably don't want to, either.)
The flight attendant will usually heat a bottle for you. Be sure that you shake it well and test it thoroughly, as the galley system often makes things very hot.