stay busy. play little games. make sure to get up and move around every once in a while.
Tabasco sauce in the eyes.
Proper sleep is the key to being alert and awake.
Plus 1. If you are having trouble staying awake, go out of service and go home. Don't risk people's lives because you're tired and next time come to work better prepared for your shift.
It is dangerous to drive tired like some are describing (running redlights because you're basically sleeping at the wheel??? Are you freaking kidding!?)