Meet Your Muscles
Regardless of where you get the information, learning to breathe from your belly is one of the healthiest practices you can develop.
Why: this is the key muscle in holding the abdomen flat. This means that strengthening this muscle not only looks good aesthetically but also protects the low back when bending and picking things up. Flattening is the ONLY action of this muscle.
Work It: I am big on the transversus abdominis. It is the deepest and, usually, the least developed abdominal muscle. My new favourite core exercise is extremely simple but wicked effective. Try it!