Can you recommend a good stomach exercise to get a flat stomach fast?

Abdominal exercise: While sit-ups can increase abdominal muscle mass they will only minimially help you loose stomach fat, if any. To loose adipose tissue work on a cardio mix (walking, running, biking, swimming, circuit training, etc - mix it up). Keep in mind that you can not target one specific area of the body to loose weight, and over time the adipose will decrease in many areas of your body (stomach included).

Takes lots of work.: For a flat abdomen 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 (i.e., running, swimming, nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.).