Tips for taking a dog on a long distances (multiple day) car trip?

You didn't mention what type of vehicle you're using. I just got back from a 18 hour trip in a car. I bought a Solid Foam Microfiber Backseat Extender from Amazon that goes on the back floor between the back and front seats. Amazon says it's currently out of stock but the link for it is: http://www.amazon.com/gp/product/B003X22GDG I then put his foam dog bed on top of that and the seat. He was clearly very comfortable and slept most of the trip.

On short drives when he's just on the back seat with a regular blanket he paces around trying to get comfortable. This is not an issue with this setup.

The second thing I would recommend deals with the water situation. I put a tupperware container that's taller than his regular dog bowl inside the regular dog bowl. I then put these 2 bowls on a towel on the back seat and put about 2 inches of water in the tupperware. The regular bowl keeps the tupperware container from tipping over and the water can splash around without splashing out. This way your dog can drink whenever he wants.

Edited to include the following website that has some good pictures the backseat extender in action. These pictures give a good example of the size of it. http://travelhintsandtips.com/road_trip.php

Have a look at Travels with Charley - may actually be useful and if not, you will be captivated anyway if you have not met it.

A 10,000 mile US road trip by one man and a 10 year old French Poodle.
When the 'one man' happens to be the awesome great John Steinbeck 'it's all on'.

I started this more as a humorous comment, but Steinbeck probably has some useful things to say on the subject - and 10,000 miles of experience.

Amazon will, of course, sell you one.

AbeBOOKs will sell you 863. From $3.48 used - free shipping in US.

I would definitely suggest taking as much familiar things as you can, I mean the things your dog is used to. It's not that you need to sacrifice your comfort and take half of your home with you, but familiar smells help reduce stress and you can use that to your advantage. So a familiar blanket or multipurpose mat that is designed for car rides and holidays are a good choice. Whenever I travel with my Joey (golden retriever) I always bring enough of his special dog food. I don't want to take the risk of not finding his food on a destination. Diet change would be stressful and unnecessary.