Get a bulkhead seat:
Get a bulkhead seat! They provide extra room. Most airlines won't assign these seats until checking in at the airport, so if you want one, arrive extra early. If your child still fits in a bassinet you can place one on the floor and let her nap in it. Or you can set up a blanket on the floor and let her nap there too. (Bring a warm blanket or a sack.) However, in some bulkhead seats the arm rests won't go up and a car seat will not fit. Also, according to U.S. Federal Aviation Administration a child under 15 years of age cannot sit at an Exit Row.
Sit in the back:
Sitting at the back of the plane is good for others because the engine noise sometimes lulls children to sleep.
Stick to sleep schedules:
Try to stick to home sleeping schedules and plan the flight, if possible, to accommodate nap schedules
Some parents recommend using Benadryl to help your child sleep. However, test it out on your child before the trip, because it makes some children hyper.
Bring a lovey:
Bring a child's "lovey" and milk to encourage naps.
Tap into familiar tricks:
If your child will sleep in his/her car seat, by all means bring it along. You get a break if you don't have to hold them while they sleep.
(Image via Flickr here)