Exercise Ball as Office Chair

I bought an exercise ball to use as a desk chair because:

1) my other chair was making me slump too much

2) if you’re gonna be sitting, you might as well be getting a small workout

… but today is my fourth day of use, and I haven’t experienced any soreness or seen any good effects. Am I doing it wrong? I’m sitting in the center of the ball with my knees at a 90 degree angle (although this took me some time to balance this morning since it was the first time I was wearing heels and trying to sit on the ball), which is where you’re supposed to be.

I’ve been able to take mini-breaks and do sit-ups on the ball, which is nice, but I have yet to venture into other exercises, which I can’t quickly pop up from should someone happen to walk by.

I guess something is better than nothing …