Have you been training for a while now, but have not yet seen any visible results. There just might be a few things where you are going completely wrong. As many people think that just doing basic sit-ups and crunches are going to get you a flat stomach, they usually just do them and wonder why no results can be seen. Getting a flatter stomach is not something that can be done overnight, but with correct exercising and eating habits it can be achieved. You just need to give some devotion to the subject.
First of all, the most important exercises are not actually the ab exercises. Correct eating habits, fat burning exercises and ab exercises are all equally important. Eating correctly will prevent you from gaining any excess fat and in the long run lose it, fat burning exercises will help you to burn calories and lose that excess fat faster. Finally the ab exercises will get your stomach look leaner and firmer. So all of these three are required if you aim for flatter stomach.
So there is not just one thing or one ab exercise you need to do to get flatter stomach. It is a combination of eating and exercising. There are plenty of different ab exercises for you to choose from and plenty of different cardio activities also. Most important thing is to keep yourself active, that is the best way to achieve results and get your metabolism running. You should do fat burning exercises at least three times a week and do ab exercises also three times a week.
Only thing for you is to choose your ab exercises and start exercising.