We all talk far too much about abdominal crunches and gaining a six pack but the key to a flatter tummy is in sculpting just one of the six abdominal muscles, the transverse abdominis which, when toned, acts like a corset - nipping you in at the waist and smoothing the stomach.
Stomach vacumming is a simple breathing exercise that you can do wherever you are. I like to do it whilst sitting at the computer, it helps with posture as well! Sit up straight and inhale as much air as possible, then exhale and pull your navel in and hold! Aim for 3 sets of 60 seconds.
Combine this core activation exercise with a balanced diet and your usual weekly exercise and you should see results.