This challenging exercise from Alison Sweeney, host of The Biggest Loser, works your entire body, particularly your core. (Yes, please.) Remember to keep your spine straight and abs engaged.
How to do it: Start in the "up" part of a push-up position, with your shoulders over your wrists, body in a straight line from head to toe, feet hip-width apart and a dumbbell in your right hand. Extend your right arm and left leg up and away from body. Slowly draw your right elbow and left knee in to touch under your torso. Extend your arm and leg back out. This is 1 rep. Do 12 to 15 reps, then switch sides.
Try this move: Plank With Opposite Limb Extension