A few sit-ups and crunches are not gonna fly you to fitness heaven. This video shows you some routines you should do regularly to produce some muscles in your stomach region. Most excercises involves a fitness ball and a medicine ball.

Remember: Fat hides muscles
Fat hides muscles, so there's no help in only training hard. You have to lose body fat as well. Therefore the need of a healthy diet and specific training of your stomach muscles are required to get the visible end result you want to achieve.