The plank.

 

 Lie face down on the floor and prop your upper body up on your forearms. Slowly lift your hips off the floor so that you are balancing between your forearms and your toes. Keep your back and neck straight and your tummy tucked in. Hold for 10-15 seconds and then return to the starting position. As you get stronger, add 5 seconds per plank until you can hold the position for 2-3 minutes at a time.