I recently lost over 50lbs and this was one of the ways I got started. I replaced my computer chair with this ginormous yoga ball. When I sit on it in a comfortable posture, my feet wont touch the floor so I have to balance. With my feet on the floor, I'm semi-standing with my bottom on the ball and its uncomfortable, so I want to rock back but then my feet come back off the floor.
You wont like it at first, which is okay because my alternative was to turn off the computer and go do yard work, which is also a pretty good workout. After a few days to a week, you'll be used to balancing on it a little, and you'll subconsciously rock and wiggle, with all of your core muscles having to work to maintain balance and comfort.
Its been pretty rugged...I weighed about 250 at my peak and sat on this for a few hours every day. Its not having to hold up as much as it used to!
This was enormously helpful in improving a bad back, helping tighten my stomach muscles and improving my core and balance, just for that one use.