I decided to replace my desk chair with a fitness ball, so I could get a little extra exercise in while doing work at my desk. I've linked an article, but my purpose wasn't to have better access to equipment (as in the article), rather to multitask.

While sitting, I will roll my hips side-to-side, pump them in a front-to-back and back-to-front motion, or do what I call "hula hoops", where I make circles with my hips. It works wonders on the abs, when done regularly.

What I like most is that it's not boring! As a matter of fact, after a while, you become accustomed to doing it and don't even realize you're getting a workout! You can't beat that!

First, you must get the right size ball. Most are packaged by height (i.e. 55 cm ball for 5'3", 65 cm ball for 5'4"-6', etc.)

Next, you must make sure your ball is properly inflated. Most come with instructions and a measuring tape to gauge proper inflation.
 
"Your thighs should slope downwards slightly rather than be at 90 degrees. But the ball should not be so tall that you end up balancing on your wrists at the keyboard".

You should always start out slowly and not overexert yourself. I've heard of a "starter" practice just to get used to sitting on the ball and to gain balance that requires you to simply bounce up and down on the ball. You'll feel a little silly at first, but it does help with stability.

Once you feel comfortable sitting, you can move on to the "exercises", and before you know it, you'll have a trim tummy and tight tush!