I do something that I call "rollouts" on the ball. I kneel on the floor behind the ball, then placing my hands clasped together on the ball...I roll the ball out in front of me as far as I can...hold for a second or two...then roll back to starting position. I feel these in my core and also my shoulders/upper back area. I do 3-4 sets of 20, and in between each set I do a set of 12 pushups.