I noticed that everyone who I know has told me they have RSI, and I had a chance to observe at the computer, had one thing in common: They have a very "stiff" posture while typing, hit the keys with sharp pokes, and also grip the mouse extremely tightly with what I'd consider a very awkward positioning of the fingers (looks like the mouse is too small for the hands.) IMHO being relaxed is probably the best way to prevent RSI.
I had problems with my carpal tunnels. I had surgery on both hands ten years ago, and the problems disappeared completely. First the left hand, then two years later the right hand. Recovery time a year each, but it is worth. Probably it's genetic. I see that one of my sons had something like a crease around his wrists when he was six years old. I feel his hands just has too tight carpal tunnels, like me.
