At the beginning it will take longer for your stomach muscles remain stretched. But as you practice this technique you will master more and more and need less effort.
1- Starting position: Lying on your back, put your hands beside the body, with knees bent and relaxes the muscles. Inhale, then exhale slowly, voting air from the lungs, makes sure the body muscles are unforced.
2- After voting the air from the lungs, you stretch your stomach muscles, trying to stomach collapses inward as much as possible. By doing this, breathing should stop.
You must keep the sunken stomach 10 to 15 seconds, then breathe gently and continue sinking stomach.
3- After breathe a little, you should not relax the stomach, breathing the need to keep another 10 to 15 seconds and again you must sink the stomach. Stomach muscles should always tense. If you were hard to keep breathing, you can take short inhalations.
4- Exhale, relax the stomach and breathe normally. Then you must again release the air from the lungs and you should sink much as you can stomach. Stretching the muscles of the stomach and not breathing push the stomach up.
This practice will achieve strengthen stomach muscles in just three weeks and you also get to reduce the size of your waist.