Bring a comfortable set of headphones if it is a long flight. Drink an adequate amount of water. Due to the regulated pressure and temperature of the cabin, the body tends to get dehydrated very fast. It's a good idea to keep drinking water throughout the flight. Minimize alcohol and caffeine intake.
Alcohol dehydrates the body, as does caffeine.
If you do have a drink containing alcohol or caffeine, be sure to counter it with an extra glass of water. Keep stretching or moving around. By moving around, you can prevent the blood from pooling in your lower extremities, thus minimizing fatigue and jet lag. People with any kind of heart problems should attempt to keep moving, as this reduces heart strain from lack of blood. Your body swells slightly due to fluid accumulation on long flights.