Strengthening the core (the back and abdominal muscles) is extremely important for proper posture and back pain prevention. A strong core helps to support the spine and keep us aligned and in proper posture. Plank pose is a great way to strengthen the postural muscles of the abdomen and back.
Be sure that your body is kept straight, neither raising or letting the hips sag. Hold this pose for at least 30 seconds. As you become more practiced in plank pose try to hold the pose for longer periods of time.