+1 to this. When I got my beemer, the seats had so many adjustments I just couldnt get them feeling "right". Every time i got in the car id adjust it and my back would still ache.
After speaking to someone else, they suggested a more upright position, like Ekona mentions, and its helped no end. No more back ache.
Ive had some back problems too over the last few years, in no way as serious as a slipped disc or damaged vertebra though so for what it's worth here's my advice. Probably more apt for Peter who had a sciatica problem, I've been told that it is absolutely critical in this type of case to ensure that your hips are not lower than your knees as this puts undue strain on the sciatic nerve, strangely this means that I have the seat far more upright than I used to ( a bit like Ekona) and guess what? No more back pain!!