It's not your mouse, its your keyboard. Check out my old setup:
That keyboard is an 80's style "Clicky" keyboard by IBM, and I *LOVE* it. Besides letting me type VERY fast (160 wpm easy) the "travel" of the keys and the solid click at the end of the travel does wonders for my fingers, hands and wrists.
In my new setup
I replaced my old PC's with new Macs, but just under the table I kept my IBM Keyboard. It works perfectly with my macs, and it lets me type very quickly.
You can find these keyboards on ebay for about $10 to $50.
If you don't want to switch keyboards, grab a Tennis ball and squeeze it for about 5 minutes every couple of hours. (Booble use to have these perfectly squeezable Boobies that worked well too). Basicly, any kind of excursive you can do with your hands will help.
Good luck!