i'm a personal trainer so i get this question about 10 times a day from ladies, my advice to you and any1 with the same question is, sit ups will NOT flatten your stomach they will only make your stomach bigger, sit ups exercise your abdominal muscles thus increasing the muscle size which will increase stomach size, the muscles that flatten your stomach are the ones that go right around your bottom torso, if you place your fingers about 1 inch above your hip bone and strane(as if your going for a No2 in the toilet) or breath out very quick you will feel these muscles tence up, these are the muscles that you need to exercise to flatten your stomach and to exercise these muscles you need to be doing alot or core stability work i.e alot of twists with medicin balls, work on fit balls. a good one is to lay back on the floor (on your back) place your legs dead straight up so the soles of your feet are pointing to the celing and rotate your legs from left to right. hope this helps.