I picked up a sturdy roller with good wheels from sears for less than $30, and it helps.
But sometimes since I am a mobile guy, there is no room where I detail for it, or the ground is bad, I bring a big pad for my knees, it is actually a semi hard floatation device that people use for swimming. It's perfect for doing the chrome pipes and other things low on the car. And even PC work.
Try not bending over, when I did that, my back hurt too. Always stagger your legs, according to Mike Phipps in his video when standing over the car or hood. If you have too, just lay down on the ground and work from there. Your clothes get a little dirty, but detailing is a dirty job. Better to wash your shirt, than have a sore back. Eventually, your muscles will either adjust, or at worst maybe you could find a desk job. But for me, even with my bad bad and shoulder, I like the physical work (for now) as it keeps me limber and in some kind of shape. Better than when I sit inside and just gain weight.
Goodluck, and if anyone else has any ideas let me know. I like the idea of limbering up and doing more ab exercises, makes sense to me. I'm hitting 40 this October, I guess using a rotary now will make my body ache a little more now than the PC.