Most of us have rounded shoulders as we spend a large majority of our time hunched over our computer. My favorite exercise to combat this is in the plank position, one arm behind the head and lifting the elbow (hand stays on head). This opens the chest and works on fighting rounded shoulders at the same time working on strengthening your lower back and abdominals.
For specific exercises to work on better posture, hire a personal trainer for a session or two. The money that you will spend is well worth fighting the pain in the neck and shoulders that you will reap as a result.