Posture can become distorted for many reasons and often becomes an unconscious way of holding ourselves and moving. Once it becomes unconscious, we are no longer able to easily change our posture so we stand more erect again (a prime example of what Thomas Hanna called Sensory Motor Amnesia).
As your postural muscles lengthen and relax with individual Hanna Somatics sessions and/or exercises, you will naturally come into better alignment. In addition, specific exercises given by your practitioner can help with your particular postural situation. Another important aspect of improving posture is the willingness to investigate and change how you sit, stand, and move in your daily life.