The old myth is that doing sit-ups and other similar exercises targeted at getting a six-pack will give you great abs. This is false. While doing those targeted exercises tones the abdominal muscles, you already have ab muscles to begin with. The only way to show those muscles and look sexier is by losing the fat around the abs. Think if it like you're a jewel surrounded by ice. The only way to touch and see the jewel clearly is to melt the ice, and that is essentially what you are doing by burning the fat on your belly.
http://www.guppyweightloss.com/how-to-lose-fat-from-around-those-abs