Run tall. Keep the body upright and the head up so the chin is parallel to the ground. Do not hunch over, lean far forward or backward, or look up or down. Running tall keeps the body in alignment, and facilitates easier movement and breathing while promoting confidence.
Keep the torso stable with the entire body facing forward. That means eliminating unnecessary and counterproductive movements that waste energy and aligning the head, hands, knees and feet to face forward. Avoid these common inefficiencies: head bobbing, head shaking, body swaying, body twisting, knees and toes pointing inward or outward, and the hands angling inward.
Stay relaxed throughout the body. It is especially important to relax muscle groups not directly involved in the running motion, such as the facial muscles, jaw, neck, shoulders, and hands. Cultivating a general sense of relaxation can have a positive psychological effect by promoting feelings of ease, comfort, and control.
Young children often have excessive movement when they run because they have not yet developed awareness and command of their bodies. Give them feedback on their movement patterns, always being positive and never critical, to help them develop awareness and control.
For ways to teach good posture, look at the activities featured in A Running Start.