There are quite a few good tips in here.
For my family we always:
Pack two coolers in the car. One for snacks and one for drinks. Then we can cut down on stops and everyone gets what they want.
Take movies or something to talk about. Once we do this, that cuts down on the length of the "boring" nature of the car-ride.
Plan our breaks in advance. We let the kids know when and where the stops will be and get them to go EACH time we stop, even if they don't want to.