If you don't already have one, get a natural keyboard. That not only got rid of my RSI, but it increased my typing speed by 10wpm to boot.
As for the mouse, make sure it's a comfortable mouse. It should be angled somewhat so your index knuckle is higher than your pinky knuckle. That puts your wrist in a much more natural position. The Logitech MX1000 laser is a good example of this (and it's also a kick ASS mouse in every other aspect).
don't sit there clicking the mouse all day with no breaks. Rotate your wrists, stretch 'em by pressing your hands back and forth. Stimulate the blood flow to the wrists/hands by squeezing a stress ball or a hand exerciser.
Something that helps many, but not all, people is an eastern herbal linament called Dit Da Jow. You can get it just about anywhere. Mine comes from the Oriental Herb Company (orientalherb.com). I will warn you, this stuff does not smell very good (think soy sauce mixed with tobacco and rubbing alcohol) and you will smell like it until after about the second time you shower
