One important point is to take breaks often - get someting like Workrave and set it to force you to stop typing for 30 second every 5 minutes and take a longer

One important point is to take breaks often - get someting like Workrave and set it to force you to stop typing for 30 second every 5 minutes and take a longer break every hour. This doesn't really affect productivity, as you can think about your code during the breaks, and in fact probably improves it. I had mild tendonitis when I was writing up my PhD, and found that forcing myself to take breaks made a huge difference.