Tuck your buttocks against the seat back. Legs should not be fully extended, but flexed at about a 45 degree angle. Take frequent breaks. At least every two hours, stop at a gas station, restaurant or rest stop. Get out of the car, walk around, even jog or do calisthenics.
Exercise fights fatigue.
In addition to exercise breaks, stop for light meals and snacks. Avoid alcohol entirely. Don't allow your eyes to become fatigued or hypnotized. Break the monotony. Turn the radio on for a while, then off.