Isn't it hard work, all this remembering to 'stand up straight', 'shoulders back', 'chin up!' being examples of the many and varied messages we typically had drilled into us as kids? There is another way:  

Your spine, that wonderful chain of interlocking bones that is both at once flexible and strong, topped by your head, pelvis at its base, is a fundamental key to better posture.  Not so much by holding them in a certain way which has to be done via conscious thought, but by awareness combined with attention to the motion of rotating it around itself.  This is what this lesson explores, gently, simply, lightly.