I had this article from a few years back from a MensHealth.com article. Good info IMO.
Suck In That Gut
A flatter stomach and healthier back can be yours in no time at all with our easy-to-do midsection tighteners
By Michael E. Price, M.S., C.S.C.S.
There you are, hanging out at the beach or pool when a hardbodied honey crosses into view. There you go, doing what guys invariably do at such times—you stand a little taller, stretching your spine, and suck in your gut for all you're worth, trying to impress her but also hoping she breezes by before you pass out from lack of oxygen.
If you're losing the battle of the bulging waistline, you will be pleased to learn that one of the most effective techniques for flattening your stomach is doing what comes naturally.
Sucking in your gut, a movement commonly referred to as the “stomach vacuum,” is a great way to work the transversus abdominis and internal obliques, muscles that give you a flatter profile and narrower waist.
As a trunk stabilizer, the transversus abdominis is very important in protecting the lumbar spine from injury. If you suffer from low-back pain, incorporating stomach vacuums into your workouts may help mitigate that pain.
Three-time Mr. Olympia Frank Zane has these tips for creating a vacuum that will lead to a small waistline:
“Lying on the floor, exhale completely, forcing all the air out of the lungs/ diaphragm with your abdominal muscles. Next, instead of inhaling, suck in your gut, pulling it in so far that you can imagine your abs actually touching your spine. Since there's no air in your lungs, you should be able to suck your stomach way in, which will contract your transversus abdominis.”
“Practice holding this vacuum for longer and longer periods of time. Eventually, your waist will get smaller, providing you don't stuff yourself at meals and bloat with massive amounts of liquids between meals.”
“The idea is to keep the waistline tight, under control. Eat smaller meals, and don't ingest as much food at each feeding, which will prevent unwanted stomach distention.”
“Using this technique, I find myself sipping water more often throughout the day instead of bloating my gut with big meals and large gulps of beverages.”
“If you want to develop endurance—that is, the ability to maintain a vacuum for longer periods of time—as well as a muscular waistline, it will all come down to breath control.”
“After you have achieved a sense of control in the lying position, next practice the standing position. Initially, it will be easier to achieve the vacuum if you press your palms against your upper thighs after exhaling and sucking in your gut.”
“The final step in doing the ultimate vacuum is to start practicing with your hands behind your neck, which is what I used to do at competitions to good effect. This particular vacuum ain't easy, but keep at it.”