Tickle the roof of your mouth with your tongue. A truck driver's trade secret, it's almost impossible to get drowsy while annoying yourself.
Eat frequently. Studies show that people who work the graveyard shift have higher rates of obesity than people who work the traditional 9 to 5. This is because in order to fight the body's natural inclination to sleep, they feed themselves to stay occupied. The same method works while driving. But instead of grabbing that corndog at the rest stop, go for something healthy like fruit and vegetables.
Listen to music you don't like. Mexican polka, doo wop, middle eastern ululating show tunes, tune into whatever sounds like fingernails on a chalkboard. If it sounds like an alarm clock, it will likely keep you up.
Get some fresh air. Open the windows to cool the car down and stimulate the senses. The roar of the wind will keep the ears perked, the smell of the grass or farms or city fumes will interact with the nose, the wind will cause eyes to water.
When stopping for gas, take a brisk walk and do some stretches. Get the blood flowing to keep muscles refreshed.
Install a CB radio in the car. Talk to the truck drivers for tips on long haul driving, staying awake, or just general banter. CB's are like mobile chat rooms.
Buy a satellite radio system. With several channels of comedy you can laugh your way all the way cross country.
