This is a very simple piano posture excercise with a powerful effect. All you do is play all the C's up and down the piano. You could even hit wrong notes - it's more about the movement then the notes. But you have to do it with an awareness of any unessential body movement. This exercise should be played with the upper body remaining still, and only arm movement to get to the extreme ends of the piano.
I use this for students that seem to feel uncomfortable physically at the piano to literally help center them. There's a lot more you could do to add to this piano posture exercise, but hope this helps as a starting point.
