When I started to study harp, my teacher also emphasized that proper hand position is VERY important to avoid injury--this is undoubtedly true for any instrument. I am a self-taught guitar player, so God knows what awful things I am doing in that regard, but I don't have time to play enough to hurt myself, and so have never addressed it. I have recently been having very severe pain in my arms and shoulders, and have just discovered Feldenkreis exercises, which have helped tremendously. The method involves very tiny, slow movements, and can be done in whatever position you can comfortably achieve--lying down, sitting up, whatever. I have only attended one class so far and only done one set of exercises, but 15 minutes in the morning stops the pain for 24 hours. There are both books and classes available--check it out.