I do something very similar to what Shelby recommends---switching positions, chairs, and input devices many times throughout the day. I stand or use my laptop while in a light stretching position on the floor while working in my home office, where I deliberately don't have a chair. In my Adobe office, I alternate between a standard chair and an exercise ball. But here's the change that made the most difference for my wrist, arm, and shoulder:

I am a left-handed mouser at work and a right-handed mouser at home. This took only a few weeks to transition into doing, and it made it so that I haven't had a moment of RSI-related pain in 10 years... after debilitating pain for the year or so right before. Spreading the stress/strain across both sides of my body means that neither side does too much in too short a time. I kept the right side for home because I'm originally a righty and didn't want this work adaptation to detract from my creative work at home.

Also, no matter how busy I am, I force myself to take a short walk after every hour of stationary work. (Phone meetings aren't stationary, as I'm usually doing some very light exercise on the other end of the phone.)