Sorry for the delay Jolk!, I meant your hand posture, or the tension that you put into playing. Try to be more relaxed, for hands I reccomend you to do this
http://datarealm.com/marbeth/relaxation.html until you feel your fingers relaxed.
For the back, you can lay down on the floor with a towel rolled and placed right under your column. That's sooo damn relaxing!.