Doing Yoga is the only answer to maintain a perfect slim body. No matter how heavy you eat food you will never grow fat. Learn yoga through a professional since in yoga inhaling and exhaling of breath parts an important function.  Correct breathing while performing asanas gives good results and stamina. Balanced circulation of blood and pains in joints etc will get removed.

Indulging in too much of sex  often leads to production of gas and bulging of stomach.   

Muscle building exercises like lying flat on the ground and doing abdomen exercises on a daily basis meticulously will certainly bring iron strength to the belly and flat abdomen.   While doing abdomen exercises  you may have to do it a little hard way with bearable pains in the stomach part.