This exercise provides a great reminder of how to sit and stand correctly. When you look at a side view of someone standing sideways behind a plumb line, you see the line passing through the center of their ear canal, the middle of their shoulder joint, the middle of their hip joint, the middle of their knee and the middle of their ankle. This is considered perfect posture with the least muscle effort and the least stress on any joints.
To do the exercise, stand with your back and heels touching a wall. Gently tuck the top of your pelvis towards the wall. Do not force your low back to touch the wall. Next, tuck your chin and keep it tucked while you move your head back towards the wall. Ideally, you'll touch it. Keep your head back with your chin tucked as far as you comfortably can while you bring your shoulders back and down (shoulder blades towards each other). Hold all these positions for a 3 to 5 count then step away from the wall and hold for a 3 to 5 count. This is much closer to where all our postures should be more of the time. Do this frequently throughout the day. With practice it becomes a great deal easier to stand straighter and more comfortably.