A chiropractor can straighten and align your spine temporarily, and help with understanding what it should feel like. The process may also stretch the muscles that need to be stretched. However, my experience with postural issues is that one also has to pay attention and help fix them oneself as well - it's easy to fall back into bad postural habits.
Based on other people I've seen, the Alexander technique seems to an effective option for improving postural issues. Pilate's can also help, but can result in a stiff looking posture.