I used to get a sore back quite a lot. As far as I could tell I was using my back too much when I was hard on the throttle, and I was standing where it wasn't necessary, ie all the time. I got around it by sitting where possible, usually for only a few seconds, which was enough to keep the back pain away, and didn't slow me down.
I also didn't anticipate well enough, meaning lean forward before opening the throttle or dumping the clutch, leaning back when hitting the brakes... letting the knees flex just before a bump. Obviously anticpating is part of riding, but now I do it much better than I used to so I don't tire as quickly - and go a bit faster.