That's not safe. Pull over, take a break, let someone else drive. Now here are some suggestions that might help keep you from reaching that point.
Have a conversation with someone. Obviously this only works if you've got at least one passenger, who happens to be awake. If you have a safe way to use your phone (hands-free, bluetooth, etc...), call a friend or family member who's willing to chat (or listen).
Stimulate your senses. Crank up your radio. Sing along. Turn your A/C to MAX and do the same with the fan speed. A nice cold breeze on your face could be just what you need.
Drink. Ok, we're talking legal beverages here. Plan ahead and get yourself an energy drink or a soda, or both. I've even found that a bottle of cold water does the trick most of the time, without the side effects of caffine.
Keep your mind busy. Play one of those "road trip" games. Hunt for license places, landmarks, letters of the alphabet or all of the above.
Don't get too comfortable. Avoid reclining in your seat or letting yourself use the headrest. Skip the cruise control and keep yourself occupied with maintaining your speed.
Any combination of these should keep you going for a while. Just be prepared to call it quits at the first sign of dozing. Don't put yourself or your passengers at risk by trying to be a hero. It's better to take a break and arrive late, than never arrive at all.
[photo: Craig Murphy, Creative Commons]