my desk chair.

Yes.  I sit on an exercise ball at my desk.  Over in the corner I have a rather darling pink & white Pottery Barn desk chair, but it is not what I sit on when I have hours of computer time ahead of me.  I work with an amazing massage therapist at The Little House Day Spa.  I happened to mention to her some time ago that the only time I feel any serious pain is when I stand up after sitting at my computer for a long time.  That is when she recommended that I try sitting on an exercise ball.  The ball "chair" forces you to sit up straighter and you automatically hold your core muscles a little tighter.

I headed straight to a sporting goods store and purchased the size that related to the height of a desk chair (when my weight is on top of it.)  I noticed an immediate difference.  And I did not roll off the ball uncontrollably as I anticipated.  For whatever reason, this past week I started using my desk chair again... until the stiffness drove me back to my handy dandy ball.  Besides, it is fun having playground equipment in your office.