This is a great exercise to develop core strength, improve posture, and build endurance in both the abs and back muscles.

To perform the plank:

1. Lie face down on the floor.
2. Prop your upper body on your elbows resting your forearms and your palms on the ground.
3. Push your pelvis and knees up off the floor supporting your lower body on your toes.
4. Keep your back flat so that a straight line could be drawn from your head to your feet.
5. Hollow your abs by drawing your belly button in towards your spine as far as possible.
6. Hold this position as long as possible, shooting for 20-120 seconds.

The plank works on the principle that the fewer number of body parts touching the ground, the more your muscles have to work to maintain posture and balance.  Therefore, if you are able to perform 120 seconds of the plank, you can make the exercise more difficult by raising one of your legs off the ground (make sure your raised leg isn't resting on the other leg).