Great advice given thus far... all suggestions will definitly play a positive roll.
Just chiming in to second Fazc's and JTurners suggestions of rearward/horizontal shrugging and pull aparts. Anything involving scapula retraction will go a ways toward improving posture.
Also fight to stay upright in your day-to-day life too. Corrective exercise will only go so far when you live the your life outside the gym like Quasimoto.