I didn't have anything as serious as tendonitis, but I did get shooting pains up my arms. Two big wins for me were:
1) make sure your posture is correct. Install a program like workrave (
Workrave) and when it reminds you to take micro-breaks, remember to sit properly and take a drink of water too.
2) give the dvorak simplified keyboard a go. it's easy to learn and a *lot* more comfortable than QWERTY, plus all systems have it pre-installed because it's a standard. A good place to look for info was dvzine.org which is now extinct, but you can find it in the wayback machine at archive.org. A good online tutor is A Basic Course in Dvorak (google "abcd dvorak")
I won't vouch for Dvorak speed claims, but it has definately solved the problem of pains in my arms. Plus because I'm actually a touch-typist now (never learned QWERTY after 15 years!) I think that has helped at least as much as the different layout, so if you're reluctant to try it, at least learn to type properly on "normal" keyboards.