Hi volks,
I have been taking my two sons on the 22 hour each way trip to
northern Idaho for the past four summers since my 6 year old was 3.
One thing that helped, especially on the younger side of things, was
to never expect more than 3 hours of travelling at a time.  Rest areas
and some run around time for 45 minutes-plus gets the little restless
bodies discharged and ready for the next round.  When travelling
without an adult passenger, I got one of those long grabber things to
pick up precious dropped items, hand napkins, etc, while driving (!)
to the back seat of the vanagon without having to stop every time they
needed something.
Singing, counting things of different colors, etc., all help to keep
them involved and pass the long hours with some fun. And a DVD player
on the rear table, with some sticky shelf stuff underneath, helps too,
though I was very reluctant to put on a movie when, for instance,
driving through Yellowstone, though the kids didn't quite share my
hesitation, of course.
-macjack