If you want better and quick results, do it more often, at least 5 times. Some people need to make little more effort than the others to keep their abdominal muscles tense. But once the muscles adjust to the exercise, you will need less effort to do it.
Here is what you need to do:
1. Initial position. First, you need to lay on your back with your arms next to your body. Knees are bended and muscles relaxed. Start exhaling slowly to release the air from your lungs. Your need to maintain your muscles relaxed.
2. Suck your stomach in. Do not breathe while you suck your stomach in. Keep it that way in that position for 10-15 seconds. Then take a small breath and keep sucking your stomach in.
3. Breathe slowly and do not rush to relax your stomach muscles. Suck your stomach once again and hold your breath for another 10-15 seconds.Inhale, and make sure you keep your muscles tense. If you cannot hold in such position for long, take a short breath.