1. Start out by lying on your back.
2. Place your hands crossed on your chest and lie flat on the floor with your knees bent. You should always bend your knees because it provides necessary support for your lower back.
Some people like to place their hands behind their head but this is not the ideal position because you can strain your neck. What happens is when you get tired you have a tendency to pull on your neck (rather than your abs) to complete the exercise.
3. Slowly raise yourself up using your abs while pressing your lower back to the floor. As you reach the top of the crunch slowly exhale. Then slowly lower your back down to the flow as you inhale.
4. Repeat this process 20 to 25 times and rest for about 90 seconds. Then perform 2-3 more sets of crunches with 90 second rests between each set and you're done for the day!
Remember, the key is not to do 500 crunches at a time. The reason is, after the first 20 or 30 reps, your body gets lethargic and you end up "cheating" and not getting the full effect of the exercise anyway.
Always do a small amount (20 to 25) controlled movements at a time. You should really feel your ab muscles working by the time you hit 20. If not, you're not working your stomach and abs the way you should. Remember to focus that movement right in your middle section.