I noticed from the date of the comments this is an old goal for you so you may have already achieved it, but just in case your still looking, have you tried Pilates? Its a great workout thats easy to do, and one of the results is improved posture. I was doing it before to get a flatter stomach and I noticed people commenting on the fact that my posture was better. Well, just my two cents and I hope that helps. Cheers :)