How to have a flat stomach without losing weight?

Abs. For a flat abdomen you must eat healthfully ; be physically active. Calories burned must > calories consumed. To tighten ab's do crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge ; “bicycling” exercise on your back. Engage in regular aerobic exercise.
Flat stomach. Besides exercise, there are procedures like liposuctiion, tummy tucks (witch hazel) and a new one called liposonix.