We recently evacuated due to Hurricane Irene and took a road trip with our three kids, a 4 year old, 2.5 year old, and 7 month old. They'd ridden 3 hours before, but nothing like this trip. Twelve hours there, 9.5 on the way home.
So you can imagine how many times my 4 year asked, "Are we there yet? Are we almost there?"
Someone recommended I try this little trick for the way home...oh, I could kiss whoever came up with this idea.
We set up a timeline in the car. We tied a ribbon (you could use string) across the ceiling of the car to the handles at the doors, making a straight line. We drove 450ish miles, so I divided the string,making a mark for every 25 miles. I used a hair clip (hey, it was last minute!) to mark the halfway point. Every time we drove 25 miles, I moved the clip. It was a great visual representation of how far we'd driven and how much was left. There was a huge cheer when we got halfway. :)
So, on your next trip, try this and see if the "Are we there yet"s are fewer and less often?