Get on hands and knees. Keep your wrists directly under your shoulders and your knees right under your hips. Extend your right arm and left leg straight out simultaneously. Bring both limbs back to starting position. Then extend your left arm and your right leg straight out. Continue alternating arms and legs while keeping your abdominal muscles tight and trying not to move your torso. Do two sets of 10 to 12 reps on each side.