I had back pain when I first started. I got a mechanic's seat so I could sit down doing sides of vehicles and also while doing wheels. That pretty much resolved it for me.
As for wrists, I have arthritis like symptoms, that precede my detailing career, doctors don't know what I have, it's not arthritis but symptoms are the same. That being said, detailing has not made it any worse for me. I do low volume, about 2 clients per week and I have a part time job as well so I work on average 5-6 days a week. My advice would be to get a brush attachment for either a drill or a DA so you can scrub carpets and seats without having to use a hand brush. Other than that, maybe go to medical forums and ask advice there. I don't know if wrist support bands or any other device could help but it's worth looking into it.
On very bad days I use Ibuprofen to cut the inflammation and pain down. Any NSAID should help with that but I feel Ibuprofen works best for me.