I don't suffer from any back issues but find having any amount of the lumbar support active actually gives me back ache. I always have it fully retracted as others have suggested, with the seat back fairly uppright, but on the occasions my partner adjusts it and I forget to take it off, I get an aching back.
You can spend ages adjusting it all to get everything just right, and then someone else gets in and buggers it up.
I am speculating that the (excessive) lumbar support may have aggravated my lumbar injury. I have retracted it fully now but use a flat thin pillow for support instead. The seat itself maybe a bit too soft, like a soft couch, which is never good for back pain. I will play around with the lumbar support later and setting it to flush, once my lumbar heals.
I am now setting the seat further back to allow my legs to stretch a bit more. I will try setting the seat higher.
The pilates is an interesting suggestion, I have heard many good things about it. Supposedly it builds your 'core' muscles around the pelvic area? When I used to do yoga (the light version not the human pretzel) it helped with the stretching and building thigh muscles. You feel a lot looser in the back and with the body feeling flexible there's less strain build up. Basically your body is not solid as a brick and you can actually bend your back!
Stretching your hammys in so far as to allow it to stretch right up your lumbar is also good for back pain. I feel kinda silly doing warm up and cool down stretches after driving the GTI. But it's all worth it to relive a bit of youthfulness right?

I refuse to drive a bimmer or audi just yet and will hold out as long as possible!
The cream for instant relief is Biofreeze. The spray form is what the professional football players use.
Cheers,