Hi CK
Practising to relax is a good thing and boosts the enjoyment of playing. I believe posture is very important especially when your sitting down. Try not to hunch your back or tense up too much. As has previously been discussed here, having a good stool or chair to sit on is big advantage.
Also, I find that keepin' the groove while practising and/or playing is a great help in relaxing. Just keeping the beat with your foot is surprisingly helpful in relaxing the muscles and keeping the hands relaxed.
Hope this was a bit of a help to you anyway!
