Only a year or so into my career I started getting pain in my upper right back from using a mouse. I tried various adjustments to my desk/chair setup but eventually it got to the point where I couldn't use the mouse with my right arm anymore without unbearable pain and it would linger when I wasn't working.

I was able to solve the problem completely by starting to use a trackball with my left arm. I still use a mouse with the right when precision is necessary but 90% of the time I use the left and trackball.