hope this wasn't srs. I made a 80% improvement to that exact posture within a few weeks. It's simple, FORCE your shoulder blades back and together ALL the time. It will hurt like fuck at first. Don't stop. Literally do that for most of the time for a couple of days and you will see a huge difference. Throw in some pec stretching here and there and do a shit load of rowing and back work in the gym.