This exercise is used to strengthen the core in a manner that it is used every day; stabilization. This is a highly effective exercise that will not only build ab muscle but it will also protect your core from injury.
Begin by lying on your stomach on the floor and resting on your forearms. Next, put your toes on the floor as if you were to perform a push up. Next, push your body off the floor so that it is completely flat and you are resting on your elbows; your body should make a straight line from your head to your heels. Your arms should make a 90 degree angle. Now, push your hips forward to prevent your glutes from sticking up in the air. Squeeze your abdominals and hold this position for 30 seconds and then rest. Repeat 30 second sets 3 to 5 times.
As you become more advanced you can increase the time of each set for up to 1 minute and beyond.
This exercise can also be done on your side to hit the oblique muscles. You can also make this exercise easier by keeping your knees on the floor throughout your set.