It is important to make sure you have proper air pressure. Depending on where you are working out determines how much air pressure you want in your ball. For those just starting out you will want to make sure the ball is not real firm. This will allow you to get use to the unstable environment. Once you are comfortable fill the ball so it is more firm. This will give you a more unstable ball causing you to work harder to maintain balance, it is great for the whole body. DO NOT OVER INFLATE!!!! This could cause your ball to burst, not good.
Safety tips: If working out on a bare floor use an exercise mat, be sure the space is void of any items. Most important follow the directions on the ball for inflating it. This is not a toy do not allow children to play with the ball as if it were. This could result in punctures and wear the ball was not meant to endure.
I wish I would have known about the size thing when I bought mine. When I was going to the gym they had various sizes and I used the bigger one and wow what a difference. Guess it's time to buy a new one, sad to see this one go we have been through alot together. I just realized I have had it for like 7 years and it is still going strong.