Some really good tips already posted, so here's mine.
While I don't have the whole 15 hours plus to drive, my 'things' are:
Book on tape (or CD) - nothing keeps me as mentally alert as following along with a story while I drive. I have actually waited in the parking lot twice now, upon arrival, to hear the end of a disk (or story itself). Getting into a good story on the drive is a major help. (Note: change the speakers to the front of the car to not disturb the backseat crew as much.)
Running calculations and notes - doing the mileage math in my head helps, as does jotting down driving notes for future trips. Having to change positions and routine, to write things down, always perks me up a bit.
Caffeine - but not at first. As a fall back, sure, but not from mile 1. It really is more of a back-up for me. Same with any 'Stacker' type product. When I have to reach for either of those, I make a quick evaluation about how alert I really am, and try to see if I am getting too tired to drive safely. In that case, I would pull over and either fuel up (noise, activity, lights), or just catch a quick nap.
Those new Listerine mist breath sprayers offer a quick pick me up. Three or four blasts with one of those and I feel like the Reverend Jim character from Taxi.
When we stop for gas, I always walk around, and stretch. A deep, almost 'painful' stretch helps wake me up alot.
The entre car usually sleeps while I drive overnight, but I have no problem waking someone up to talk to me, if I feel my alertness is declining. On occassion, I will let pull over and ask my wife to drive. But, she too is not fond of driving at night, so usually this is a quick fix so I can catch a bit of sleep (an hour, maybe 2), while keeping the car moving towards the Mouse.
And lastly, my super duper tip. Drink everything in the car you can reach! With or without caffeine, if you drink enough, you will have to go to the restroom, and I cannot sleep or get very comfortable if that is the case. It's almost like tricking yourself into stopping for a break, in the very immediate future.
Always be prepared to stop somewhere, as a fall back option. Remember, it's better to get there late than not at all.
Drive safe.
Robert P