The Side Plank is a fantastic, isometric, core-conditioning exercise. By planking on both sides of your body, you'll reduce any imbalances between the two, but strengthen your abdominal muscles as well.
To begin: Find a comfortable mat and place it on the floor. Get down in a plank position with your elbows supporting your weight on the mat and your legs extended out behind you.
Keeping your body linear and stiff as a board, twist your torso to the right so your left elbow stays on the ground and your right arm is extended out and pointing to the ceiling. The sides of your feet should be the only part of your legs touching the ground.
Keep your core tight, and hold this position for 30 seconds or more. Once finished, reverse the position so your left arm is directed to the ceiling and your right elbow is holding your torso off the floor. This counts as one set.
The key to performing a Side Plank correctly is to keep your body in a static position while your abdomen and back maintain a neutral spine and your elbow supports your bodyweight above the ground. You are simply staying prone and holding the position for as long as you can manage. Try to avoid sagging your hips or spine throughout the plank.