I always fall asleep when I drive, even if it's just a few miles home from work. I've developed ways to stay awake. Here's how.
Realize that you are falling asleep.
Turn on music and start singing to it. You'll look like an idiot of course, but it's better than looking dead from an accident.
Chew gum. Don't just put it in your mouth, make sure you are chewing it.
Call a friend. Talk about silly things. Find a friend that let's you do the talking, instead of just listening. It won't help.