This is one exercise that is all too often performed badly. Remember, for the exercise to be effective, it must be done correctly. Follow the guidelines below and you should feel your abdominal muscles working hard to hold the position.
Start with your forearms on the floor and your elbows slightly behind the shoulders.
Tuck your toes under and lift your body off the floor.
Activate your abdominals and squeeze your bum.
Take care not to let your lower back arch or your upper back round during this exercise.
If you are able to hold this position with good form for a minute, you can increase the challenge by adding alternate leg-lifting. Perform these by squeezing your bum and lifting your leg 6in from the floor. Ensure that you keep your belly button drawn in when doing this.
Done right, this is a hard exercise. To reduce the intensity slightly, perform the exercise as above, except with your knees on the floor.