When your abdominal are strong,your posture will improve.When you sit,suck in your belly (pretending you are pushing belly button to you spine) about 10 %.Try to keep it that way while you are siiting or doing something else.Doing some exercises(like pilates) to strengthen you abdominals will help.

When you have bad posture or abnormal curvature of the spine, if you have had these problems for some time the vertebral joints tend to become fixated.

Its believed this is because the vertebral joints are encased in layers of soft tissue such as ligaments, fascia, (sinew that covers muscles) ,and muscles that shorten ,and tighten, its believed that these types of conditions respond best to manipulation ,and mobilization performed by chiropractors osteopaths and physical therapists if exercises alone don't improve your posture i would consider this type of treatment from a good practitioner just like surgery be careful who you let treat you.