Richard, one stupidly simple trick that might help is to add a different type of motion. You've probably found that standing still is hard, and walking is more tolerable. It is possible to mimic the movement of walking. Also, when standing still, it can help to flatten your back.
What's the stupidly simple trick, you wonder? Place a box, short step stool, or something supportive in the 3-6" height range, maybe 18" wide, under your lathe, an inch or two ahead of where your toes are. When your back starts to bother, put your left foot up on the step. In a little while, your back will start to bother again, so switch to your right foot. If the switching is frequent, this can be like a walking motion. This trick isn't going to work for everyone, but it costs next to nothing to try and see how much benefit you get.
There are also strengthening exercises for low backs (NOT sit-ups or crunches), which if they aren't already part of your daily routine, along with a walk, you need to add. These can be really helpful, if properly done, and many people should learn them from a physical therapist to do them properly and not cause problems. If that's not feasible, look up Mackenzie Back Exercises on the Internet, as they're a safe, DIY alternative.
It also might be helpful to have your club mentor watch you turn, if you can. It's possible that your feet are positioned in a way that's adding to your trouble, and a mentor might notice and recommend a change.
www.woodturner.org
