We have made several long trips similar to what you are talking about with our kids. In addition to those mentioned by Kate here are a few other helpful hints.
We did have pretty good success with travelling at sleep times or in the evenings. A few times we left in the afternoon and would then stop for dinner time and put the kid(s) in their Pjamas and give them their bed time toys.
Don't expect them to sleep . Our son stayed awake until about 1 hour before we arrived at the end of an 8 hour journey.
We have a bag with a number of toys and books so that we are not continually picking up toys. I try to keep it to toys that will not cause too much damage if they did fly around the car.
I like to know how far it is to the next decent stopping point. Can you put up with the complaining until then.
I organize lots of small snacks. My son will often eat a lot while we are travelling. So we try to stop at places where mum and dad can have something to eat and/or coffee and the kids can burn off energy. My son is older now (4) so I say that when we next cross a bridge I'll give you something to eat or when we get to a certain song on the CD.
Expect at some point that your child will lose the plot. An older woman gave me some good advice when our son was tiny she told me when this happens have some music you enjoy and turn it up. My husband and I now will sometimes listen to our MP 3 players to drown out the kids. Realise that the more times you stop the more longer the trip. I did spend 30 mins one time popping a toy over the front seat into the back. It was the only way to stop our son crying.
In warm weather especially take lots of water for them to drink. My son would drink about twice as much water as normal when we are travelling. This was something I didn't plan for one time.
