If you really have RSI, the most important thing to do is:
Stand up every half an hour!
You absolutely have to get up for at least three minutes, at least after every hour (better every 30 minutes).
Do you know that feeling of dizziness when you get up from your chair? That's when your metabolism was geared down to neutral. When you stand up, the low blood pressure drains your brain, and your heart quickly wakes up to level the flow. It's a clear sign that you had been sitting and typing for way too long.
There are very nice typing break tools out there, for example blanking the screen for three minutes every hour. However, that makes little sense unless you actually also stand up during such a break.
Better yet: walk around, make some tea, wave your arms, dance, jump, run in circles laughing madly (take care not to knock over any furniture).