I second what Irate said. That was dead on. Dont forget about your back, but if you ignore your hamstrings, you'll NEVER correct your posture. I gather from your posts you are a student and probably sitting at a desk for most of the day. Having your knees flexed in a seated position all day long, repeatedly, will cause your hamstrings to shorten. In turn this tilt your pelvis posteriorly. This affects the curve in your spine and will eventually lead to stooped posture.
Great advice for your back in this thread so far. But if you forget about your hamstrings, theres only so much improvement you will realize.
This looks like some good information:
http://physicaltherapy.about.com/cs/.../aa081501a.htm