I have Carpel Tunnel and have for many years in my right hand & tendonitis in the left. It is all part of being a guitarist. The best thing I do to prevent it is to always warm-up gradually, and to play at a correct position whether it is standing or sitting. Like do not hold the neck up with your left hand and make sure your right hand is not up really high, find the best spot. Find away to play that is not uncomortable. Any sign of pain means you are pushing it. If I take it easy and warm up and try do things that I know agravate it, things are good.