I always have a water bottle with me and when I get really really desperate to keep DS1 awake, I spray him with it. I know, it's sounds like I'm treating my son like a pet, but it's generally safer than trying to reach to the back and shake him while I'm driving. I've even tried to make it a game when I know he's really sleepy and have given him a squirt gun to squirt me back... granted, I live in a warm climate and don't have to worry about his clothes freezing the second I take him out of the car.
