this might help you out-
http://www.mayoclinic.com/health/core-strength/SM00046
My PT (who has personal experience with PT & MS) suggested these to me the other day as a good beginner's set for me:
sit on the ball while:
working at computer/work
eating meals
watching TV or chatting on phone (increase time on the ball every day or so)
Just doing those 3 made me "feel it" the next morning and I didn't even think I had been working to stay sitting...
*but I did fall off my ball while eating my breakfast - it was hysterical-- me on the floor with a "what the he**" look, covered in my frosted flakes, mobbed my cats going after the milk; all while my husband asks "Honey, why did you do that?"