[Mahler]

You didn't listen when mom said, "sit up straight."
Good suggestions so far, but practice good posture whenever you can.
When standing in place, such as in a line, I have often adopted a military rest stance with hands behind my waist and legs slightly spread. When sitting, especially in a hard back chair, I often sit a little forward and up straight without using the back rest. When driving, I adjust my rearview mirror just a tad higher to force me to sit up straight to see what's behind. As has been said, once you start to correct your posture, you have to follow through and practice it.