Drinking water, doing cardio exercises, and crunches can help flatten your belly. Drinking water breaks down fat for energy and hydrates your muscles. Cardio exercises like running are effective for burning belly fat. Crunches work the abdominal muscles. Improving your posture by relaxing your pelvis and angling your backside back can also help reduce belly size. Eating smaller, more frequent meals instead of three large meals can avoid filling up your belly and taxing your digestion. Reducing high-fiber foods that can cause bloating provides additional tips to flatten your stomach.