Belly muscles are designed to give balance and coordination for the body's movements. Constant movement keeps it in shape. People used to walk and move a lot just a few centuries ago. These days most of the people sit all day long at the office and as a result their muscles weaken. At this point they can't support the internal oranges, allowing the abdomen to bulge.
The swelling causes people to seek some drastic action, in order to regain that nice flat belly they start to exercise. This is where people make the biggest mistake in flatting their stomach: they start to workout very hard on their belly muscles and develop nice firm muscles but no one can see them. They are all sunken under a layer of fat. People are walking around with nice strong belly muscles that are actually invisible.
These people can not stop complaining about how hard they work, every day shaping their muscles but no visible results are shown. This is their mistake, their muscles are just fine. The problem is the layer of fat which covers them.
The solution is working on both things at the same time: on one hand work on your belly muscles, in addition you must reduce your fat percentage, Lowering your fat percentage will eventually give you the affect you were looking for the "six-pack" look. It is very important to realize that lowering only belly fat is impossible. You must burn fat with by working and developing a higher muscles mass which will burn more calories even when you rest. Another important aspect is to reduce fattening food consumption.
There is an argument regarding how much and how often should one work on his belly muscles. It depends on you, your time, and how quickly do you want to start shaping your belly. It is recommended to work once in two days for 15 minutes, work real hard the n rest the next day. It is possible to work every day on the belly muscles, contrary to other muscles, but its best to start in this moderate manner and move from there.
This is it the big secret do not expect to find specific exercises, it is not the most important thing the key to win your fight for a flat belly is in the combination of low fat diet and exercises which will end up in a flat belly.