Weirdly I fixed my posture when I improved my voice, because The Voice Book (http://www.amazon.com/dp/0571195253). I think he covers posture in the first or second chapter (posture hugely affects your voice). It's been a few years since I read it so don't remember the exact guidance - it involves lying on the floor, getting your head aligned with your body, and ensuring the small of your back is the right distance from the floor. That improves breathing and airflow and then voice. I significantly (noticeably) improved both my voice and my posture.