Go to sleep. You might manage three or four hours, which is a big help in fighting jet lag. Don't take sleeping pills. When you're about an hour away from arriving at your destination, freshen up in the rest room. If the airline is passing around a light snack, take it and enjoy it.
Many airlines favor high sugar snacks to jolt tired passengers awake.
Flight Isometrics Imagine a tennis ball in the calf of your leg. Using leg muscles, squeeze several times. With legs straightened, point the toes up and down vigorously. With the foot just off the floor circle the foot, first in one direction, then the other. To relieve aching and swollen feet, place the foot firmly on the floor, now pull the toes strongly towards the heel, arching the foot.