Beginning Plank - Bent knee version
From a sidelying position, with both knees bent, prop your body up on one elbow as shown and lift your hips so that your body is straight (like a plank!). Throughout this exercise maintain a light contraction of your abdominal muscles. This is called "bracing" and isn't a hard contraction, but simply "firing" those muscles so that they're being the stabilizers they're anatomically designed to be! You should try to breathe normally while you hold this position for as long as you comfortably can. When you feel your body fatiguing out of position (either due to shoulder or other fatigue), lower your body back down, roll over and do it on the other side.