These seats are usually only available at check-in. Be sure to bring a comfortable set of headphones if it's going to be a long flight. Drink an adequate amount of water. 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 and 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 problem should definitely attempt to keep moving, as this reduces heart strain from lack of blood. Your body swells slightly due to fluid accumulation on long flights.