We just made the drive from Central Illinois (Hi, ScottNBecky!!

)
All the answers have been great so far. Get up frequently, stretch your legs, combo of fast food and healthy snacks in the car. PSP and stuff are great, but keep a few surprises back (like electronic yatzee or battle ship or tamogachi--just something different)
I have a few suggestions:
Take the detours!! Drive through scenic parkways, go to roadside attractions (your kids will never see the world's largest ball of string otherwise..), and enjoy the drive. During this last trip, I ate at the birthplace of KFC, a place featured on $40 a day, saw some beautiful snow in the great smokey mountains, walked on the beach by the ocean, went to a winery and a bourbon distillery. All non-Disney fun that made the trip as a whole that much better.
Books on CD are GREAT. Harry Potter is just about perfect. I know that DVDs are popular too, but you can always split it between the two.
Take glass cleaning wipes and air freshener. It's amazing how much schmutz accumulates on the interior windows of the car with all those mammals breathing in there. After a while it starts to smell like the monorail (AKA Ape House).
One other thing I would suggest is to pack a bag of plastic grocery bags. I always keep a few in the car and am surprised about how often i need them. Especially with a potty trainer or diaper wearer around.
Look for places you have never eaten before. Chikfila, Bojangles, Popeyes and Picadilly are all treats for us because we don't have them around here. And there are always back up snacks in the car if it's awful.
FINALLY--Watch "National Lampoon's Vacation" while you pack. When things go wrong on your trip, reflect on it and remember it could always be worse.