You should check out some of the ideas here,
http://www.momsminivan.com/index.html. We've used a number of them.
Our kids are a little older now (DD8, DS6) but we've taken long trips for years and the key is giving them something to look forward to through out the trip.
We always make extra copies of the route map (usually from MSN maps for us). That lets the kids keep track of our progress.
On our last trip to WDW, DW made up surprise packages to give out as we crossed into each new state. The packages were all small things and were a combination of items for use in the van (books on cd, travel games, ...) and things for when got to WDW (disposable camera, glow sticks, ...). With each package was a little information on the state that our oldest had to read.
We take turns deciding who gets to pick the next cd. Again that gives the kids something to focus on and anticipate.
The kids have their own seat back organizer that has all of their drawing stuff and travel games so they can manage them themselves.
The other thing we have found helpful is that we always pick a hotel with an indoor pool. Right after we check in we always go for a swim to burn off some energy.
Unlike Charles neither myself or DW are good late night time travelers so we leave early in the morning. The plan is always that the kids will sleep until we get to the border but it never works out that way. It does mean that they have a nap in the afternoon which also makes the trip go easier.
Don't be in a rush to get back in the car at rest stops. We usually kick a soccer ball around or do some other activity for five or ten minutes before continuing the trip.