Certainly good idea to do as suggested in the previous posts and work on strengthening your core muscle groups plus losing any excess weight if needed. Going to the gym is not always necessary, sometimes just toning the muscles through various light exercises and stretches makes all the difference. This is assuming the pain is not from degenerative discs or other underlying spinal issues. If you are otherwise healthy, consulting a physical therapist would be a good idea since they can help balance the muscles. Back braces are good for adding support, but will likely further cause the muscles to become weaker since they will not be used as much for stabilizing the body. I don't see this happening for a weekend detailer, but more for folks that wear them 8+ hours everyday for their job, whether they need the belt or not.
Another issue you may find more appropriate is to evaluate your work positions from an ergonomic perspective. This could include raising the car for the horizontal surfaces to eliminate bending, either by using some sort of lift, or on the cheap by making a series of stepped pieces of wide lumber that can be driven upon like ramps to raise the car 6-8 inches or more. If you can do most of the work standing up with the tools at the correct height without over-extending arms, at the end of the day you will feel much better and be less fatigued. Without a lift that will be hard to do for the sides, but next time you detail, take note of your working positions.
I'm about 6'3" and certainly understand back pain after detailing from working on lower slung vehicles. It's too easy to bend or twist a certain way that you may regret the next day. But most the pain I get is just from actually using the muscles, so they get a bit sore from the workout... part of the inherent problem of sitting behind a desk everyday. Never had issues when I was standing on my feet all day in a previous job... so basically, my current lifestyle makes me more prone to having problems after detailing. Detailing is really only a trigger to expose a weakness in the body, which is why I don't see the back brace being anything other than a band aide. Try to focus and resolve the underlying problem and you will be happier for it... just my two cents from learning to live with and manage chronic back and neck pain following a car accident 5 years ago.
