here is a lifetime of ballet re-iterations:
imagine the top of your head is attached to a string which goes in a straight line through your body to the bottom of your pelvis. then imagine someone is standing on your roof, tugging that string up.
downward dog is good for posture too. take an ashtanga yoga class for personal-adjustment things, because it can be a very deceptively simple posture that is easily done without proper alignment.
when walking its good to visualize your pelvis as a bowl full of water. Then your goal is to keep the bowl level, to prevent the water spilling out from the back or front. (this keeps your belly from protruding out, and your bum from popping)
like this: http://www.courses.vcu.edu/DANC291-0...0alignment.jpg
wow just repeating this and my posture is better.