Flattening the Stomach While Exercising

You can strengthen one of the most important and often overlooked abdominal muscles -- the transverse abdominals -- while sitting behind the wheel of your car. These muscles, acting like a corset, keep the organs from bulging out of your stomach and they help support the spine.
Supine Hollowing
This exercise is traditionally performed lying down but can be done while sitting in your car. Keep your shoulders down and back, but do not arch your lower back. Breathe normally but at the end of your breath do both of the following actions simultaneously: contract your Kegel muscles, which are the ones that you use when resisting the urge to urinate and draw your navel back into your back as far as you can go. Hold this position for a few seconds and release.
Seated Pelvic Tilts
This is modified from the standing pelvic tilt. Inhale through your nose. While exhaling through pursed lips, pull your navel back into your spine. Press your hips forward, with very little movement in the legs, as if you were trying to tuck an invisible tail under. Hold for a few seconds and slowly release.
Elevators
Try to picture your abdominal muscles as a horizontal elevator. Inhale deeply, allowing your lungs and abdomen to expand. Picture this position as the first floor. Exhale through pursed lips and draw your navel in toward your spine; consider this the fifth floor. Now expel the last bit of your breath out in five squeezes, pulling your navel in even further; this is the sixth floor. Release.
Basic Contractions
Inhale through your nose, but only allow your lungs to expand halfway. Exhale through pursed lips while pushing your navel to your spine. Contract your abdominal muscles and hold for a few seconds. Release.