Take an entertainment bag. I spend more time and thought on this item than on any other. How elaborate your entertainment bag is depends on the length of your trip, how many children you've got to entertain, and their ages and attention spans. You can really burn some rubber while the children wait for their photos to develop. In my family this is easy.
Some parents prefer to switch duties during the course of the trip.
If you have space, the kids love having a grown-up sit for a while in the backseat with them. Keep the peace. Putting siblings in separate rows of a van for a leg of the drive can also help. If they're still squabbling, suggest stopping the car. You can't do it in a plane or on a train, so take advantage of the fact that with a car you can pull over and get out.