Blow It Out, Suck It In
When doing crunches, sit ups, or any other abdominal work, you should always blow out forcefully at the top of the movement and suck your abs in for a full contraction.
In order to flex your abs and contract them fully, you need to blow all of the air out of your lungs and abdomen. When you do this your abs will experience maximal contraction and will actually work harder to perform the exercise.
For a slimming isometric exercise simply stand up straight, tighten your abdominals, blow all of the air out of your lungs and abdomen, and hold the contraction for 30 seconds.
Do this for 5 minutes a day every day, and you will actually see results after a couple weeks.
Do this all day every day, and you will actually train your abs to stay tight and flat permanently.