Get a good night's sleep and the night before. Contrary to popular belief, staying up all night before a long plane ride is not going to make it easier to sleep on the plane--it's just going to make you more tired, cranky and more prone to getting jet lag.
Eat a healthy, filling meal the night before your flight and eat something before you board as well. If you starve yourself before a flight, you will just feel terrible on the plane. Airline meals have gotten better over the years, but they are not always offered and they are also almost always not filling at all.
Get out of your seat once in a while and sometimes move your arms and legs when inside your seat, especially if you will be flying for more than three hours. Go the lavatory, splash some water on your face and you will find that you will feel much better. It is not only uncomfortable to sit in the same position for a prolonged amount of time, it can affect your blood circulation and cause blood clotting.
Stay hydrated. Drink a lot of water on the plane--it is offered for free. If it is not offered enough, ask the flight attendants to bring you more. Do not overdo it, but bear in mind that the atmosphere in an airplane is very dry and will dehydrate you, which will in turn make you feel very uncomfortable and, again, increase the chances of jet lag.
Tune out unwanted noises, such as baby cries, with noise reduction earphones and earplugs.
Read a long book or two or watch some movies on a video iPod or mini DVD player. If it's exceptionally boring, you may fall asleep and a good slumber is by far the best way to pass the time on a plane.
Wear an eye shade and bring your own pillow if you want to be more comfortable when you sleep. The thin, scratchy pillows airlines supply are often insufficient.
