i am a physical therapist with a special interest in back and neck problems and have treated many patients who c/o increased pain when driving. generally speaking i'd recommend highly trying to obtain the correct posture as much as possible when driving. doing so may not always completely abolish the pain, but it likely will lessen it and make it more tolerable to drive.

i believe a few people have mentioned the lumbar support, ie. placing a pillow or roll behind your low back. this would be a good starting point. however, it is absolutely crucial to make sure your butt is smack back against the back rest first, then while keeping your butt there, lean forward so that you can place the roll/pillow behind your low back. then lean back so that your shoulders are supported by the back rest.

this will help maintain your natural lumbar curve while driving, as long as your butt does not slide forward.
of course, there are a number of other factors to consider, many of which you have already touched upon here, but this is almost always my first piece of advice for any of my patients who c/o back pain or worsening back pain while sitting or driving. please be aware that too large of a support will also cause pain, as it will force your back to excessively arch. just large enuff to support your natural arch.

and i do agree with the chiros...if you do have an existing back problem, very often sitting is an aggravating factor. so make sure to get it checked out. just fixing the sitting problem is not gonna fix your underlying back problem.

for more info...try google for "ergonomics" or "car sitting posture".

one last thing...car seats generally speaking have poor back and neck support or poor ergonomic design. usually, they are built for "comfort", aka Lazy Boy recliner syndrome, which is usually not good for your back, as there is no support whatsoever or the support is in the wrong spot. they often, in fact, support the complete opposite of "upright" posture, which would be "rounded" posture like a "C" shape.

also uncomfortable is not necessarily a bad thing; it could mean getting used to a new posture, in which case it will improve and eventually abolish with time.

 

here are some links for 3rd party aids....

http://www.reliefmart.com/adjustable...t_supports.htm
http://www.sitincomfort.com/backrests.html

Despite my own bad back, I haven't tried 'em, but the other threads I saw that recommended these had good things to say....

 

http://turnfast.com/tech_driving/driving_seating.shtml
also a spot on MINIUSA with good info in the Motor On section (driving techniques?)

 

Just to chime in with a couple of my pennies, besides lower back pain, I have had some bad sciatica while sitting in my wifes car (Subaru Legacy GT Wagon) for long drives. Not sure if the peddles are a little offset or what, but i have to really take my time to make sure I am sitting just right in those seats - square to the wheel, not too far back or forward, butt up against the seat back etc. I often put a little memory foam pillow just above my butt at my lower back. Also have found that when sciatica flares, a pillow placed outside thigh and hip adds support and lessens the pain.