I'm no expert on this subject, but my other half is (he has formal training in massage and body work)... I'll try to pass along some of the things I've learned from him.
a HUGE part of fixing the problem is being aware of it and making a conscious effort to hold yourself differently and to work on the muscles surrounding the problem areas.
I have huge problems with my hips and shoulders, and when I spend the time to pinpoint problems and work on them - I look SO MUCH BETTER... the feeling better comes slightly later - the new, correct posture will not feel good/right at first.
My (completely non-professional) advice is to see someone who specializes in massage or chiropractic or rolfing or? - have them help you work towards the body and posture you want - most of the work is on you, but I know I needed some adjustments from an outside party before I could make the changes myself.