One thing that helped me was I made a foam cylinder to put against my spine in the car seat. This then helps to keep my shoulders back WHILE driving. I also try to practice using my back muscles to pull them back WHILE driving.

Note that car seats are the worst because they are "bucket" style i.e. curved to create a rounded shoulder and thus the compression bad for TOS (in those where this is a problem...and some people's TOS is not apparently affected by this posture). I cannot over emphasize reversing the rounded shoulders. More work on a foam roller and strengthening back muscles. And yes...until then...stop every 20-30 minutes and walk around. I can now drive for much longer periods without having to stop.