I had this problem when I first got my car. Took me ages to get it right. It is at least partly because the seating position is so different to a normal family car - your legs are stretched out rather than in a normal sitting position, which stresses your back muscles in unfamiliar ways. Couple of things I found that helped me get it just right, though if you are very tall only some of this may work :
- sit so that my legs go with the seats rather than fight them. This means they are closer together and stretched out more as I am not pressing firmly up against the side of the bolsters with the side/base of my thighs. They are instead holding me in.
- seat is back far enough that my foot rests easily flat on the footrest, front of the seat tilted up a bit to support any bend in my leg under the thigh. I have short legs, so pulling the support out makes it uncomfortable
- I am only 5'7", but I have my seat all the way down. This means my legs are as supported as possible in a relaxed position rather than held somewhere between a normal driving position and flat, reducing stress on my back
- my seat is quite upright - makes the angle between my back and legs smaller, less stress on my back. I saw a programme once of a Z4 test driver who had his seat upright enough that he was sat fully against the seat back but he could reach his arms out and break his wrists over the steering wheel. I have short arms, so am not quite that upright, and my wrists sit on the front of the wheel
- bizarrely I have two memory settings - one for work/suit shoes, and one for my trainers. The latter have no heel as such, and so I need to sit just a bit further forward. Totally weird, but that is how much I buggered about
I was devastated when I got my car and my back started aching so badly - my dream car was turning into a nightmare, as I drive it every day, sometimes quite long distances. However all the above helped me loads, and I only tweak it a teeny bit occasionally now - recently I drove 1300 miles to Scotland and back, with a 280 mile hoon in the middle, over 3 days... I was mentally and physically tired, and my back was a bit stiff, but it was not painful at all.
Hopefully at least a little bit of that essay will help... If only to reassure you it can be overcome