The position you adopt when you're in a sitting or standing position makes a massive difference to your musculature and skeleton. If done incorrectly, you can cause yourself major problems. In order to get a slender look, a flat stomach, a harmonious figure, you must strengthen your abdominal wall.
Several solutions exist: sit-ups, swimming... But even just standing straight is sometimes enough to improve things:
Stand tall! For example, when you walk, try to contract your abdominals and keep yourself upright. In the same way, when you're in a sitting position, it's important to keep the bust straight. You'll soon notice some easiness when it comes to retaining the right position and you'll see that your belly become flatter. If as well as standing and sitting in the correct position, you do some posturing exercises, you could even treat yourself to a couple of chocolate bars!