Get separate mouse and keyboard wrist rests.
Use a vertical mouse instead of a traditional one.
Use more than one type of mousing device and switch up from time to time. Graphic tablets with digital pens take some getting used to but are excellent.
Look up RSI forearm stretches and take breaks to stretch your arms and hands for a few moments at least every few hours. I'm also a big fan of gyro ball exercisers.
Get an ergonomic chair with movable arm rests that can keep your arms at around the same height as the desk your working at.