If your ab workout sessions are spent crunching away on the mat, try sitting and give an exercise ball a try. Exercise balls are great for balance your core muscles. The use of an exercise ball also allows you to work your muscles through a larger range of motion thus targeting more muscle fibers.
So what will this type of training mean for you? How about firm, tighter abs, rather than small bulging muscles closer to the center of your abs. When you perforn crunches you focus in on those muscles in the center of your abs instead of focusing on a larger range of motion using more muscle fibers.
When training on an exercise ball keep in mind the importance of breathing while performing your exercises. This is a common mistake most people make.
So, if you are after that thinner waistline, rather than torturing yourself through months of dieting and performing your ab exercises on the floor, try an abdominal exercise ball.