I'd suggest regular stops, so your muscles don't stiffen up and you get out of the car for a while.

Take a comfy pillow, make sure you are comfortable in the car and that you sit in a good position. Maybe lower the seat a bit so you can lay down and sleep a bit? If you can sleep through a large portion of it, maybe that's better?

I hate the car (I am only a passenger, am no longer allowed to drive), mostly because I feel the bumps in the road and the vibrations. It's painful.