http://www.selfgrowth.com/articles/Adams14.html
"If you have low back pain that never seems to resolve completely, no matter what you do for it; if you have neck and shoulder pain that comes and goes, you may have Bucket Seat Syndrome. More people are hurt by their car seats that they are by not wearing seat belts. You may go years without an auto accident, but you spend every day sitting in a seat that, down the road, is guaranteed to cause you pain.
When car makers set about making the bucket seat they must have taken their idea of the average driver (male, 5'11? 170 lbs., who slouches badly) and designed a seat around him. Ever afterward, we've all been forced into a position that only he finds comfortable.
Today's car seats, particularly buckets, position your knees higher than your hips. This throws all your upper body weight back onto your gluteus maximus and piriformis muscles through which--and this is the important part--the Sciatic nerve runs. Sit on that nerve often enough and long enough, and add a fat wallet in your back pocket, and you will probably end up with shooting pains down one or both legs.
Human beings were designed to sit on their pelvic bones, or ischium, those hard bones you sometimes feel when you first sit down on a hard chair. Sitting on those bones automatically gives us a natural arch in the small of our backs. When we sit this way, the Sciatic nerve, Sacroilliac joints, lumbar vertebrae and hips are unencumbered and unstressed.
In order to take the pressure off your Sciatic nerve, you must drive with your thighs parallel to the floor of the car, your hips at the same level as your knees. If you can adjust your seat to be flat, great. If not, fold a towel, use a small cushion, or buy a foam wedge to place in the dip of the seat.
Sitting erect in a bucket seat, even with a wedge cushion, is not easy. You'll need lumbar or low back support, too, to keep from leaning back and causing neck and shoulder pain. Once you've positioned yourself as if you were on horseback, slip a small rounded cushion or rolled up towel behind the small of your back.You don't need to lean back. Your lower body will support your upper back and head. You may need to bring your seat closer to the steering wheel.
At first, this new way of driving will feel awkward at best and uncomfortable at worst. Persist. Soon you will find that chronic low back pain, stiff neck and shoulder pain will get better and better."