[prirun]

A key thing to avoid tendonitis / carpal tunnel syndrome / repetitive stress is to make sure that while typing, your wrists are not bent. I would get a couple of wrist supports, the kind that ensure you can't bend your wrist, and wear them 24/7 until your tendonitis calms down. You may have to cut down on work to let it calm down. Another tip: do not stretch out your hands while moving fingers; this causes stress. Move your hand instead.

As others have said, ergonomics are a big deal. Check out some videos on correct piano posture; it's similar to the typing problem. Basically you want everything at near right angles. For me (mainly neck issues, not wrist), I put my Macbook on the desk, adjusted the chair height so my forearms were horizontal, and put an external monitor on a tall pedestal so that I could look straight out instead of having to bend my neck down all the time to look at the laptop screen. I don't use the laptop screen at all (it's blanked out).