We do a map that the kids to get to mark off to show where we've been and how far we still have to go. I laminate it (or even put in a page protector), and use 2 colors of wipe off marker - or just erase it and start over.
We do snacks/toys/coins/markers for every 100 miles or every hour or whatever increment you want. That way, there's something new coming all the time.
DVD's are good

- but in our car, we usually can't really see our player until evening, so we can't count on them during the brightest part of the day.
Books on tape. We LOOOVE listening to books on tape. Hank the Cowdog is a great one, or any favorite story book authors you love.
Nintendo DS or other hand-held game systems have been a savior for us, not only on the long trips, but even for some shorter trips at home or at dr's offices. We don't use them to keep our kids busy all the time, but there have abeen a few times that they've really come in handy.
Personal MP3 players. They're really cheap at electronic stores now, and while you can't do things from Apple on them, you can get buy songs from lots of other places now, or record your CD's at home and convert them to MP3 and put them on the players. We also do our audio books this way so we can listen to our own stories if that's what we want to do.
Momsinminivans has great ideas for lap games for kids, car games, etc. It's a great site.
One of our favorite toys for the younger set were half baking sheets, the kind with the taller lips than typical cookie sheets that are sans lips. They contain things like cars, magnets, etc. You can use a wipe off marker to draw game boards, play, then get rid of it
