I've been using a computer daily since the early 1990s, and inevitably some parts of my body have reacted badly to this. My right wrist started hurting sometime in the 2000s, and last year that pain extended further up my arm. RSI had become a problem that I needed to deal with.
After a bit of digging around various ergonomics websites, I decided to try a new mouse. It was my mouse arm that was giving me grief, so that seemed to be a good place to start. On the recommendation of a fellow geek, I decided to try a Vertical Mouse from Evoluent. It's a weird looking contraption, that is basically a mouse on its side.
The idea is that you twist your forearm less with a vertical mouse. Your arm remains in a neutral, natural position when using it. I was dubious this would make any difference.
However, after a week using my first one at home, I bought a second for work. I've not had wrist or arm trouble since. It has changed my working life. I wouldn't go back to using a normal mouse.