Here a few things to do:
1. Massage – Get the soft tissue of your back (upper and lower), shoulders, chest and hips done. Get a foam roller and/or tennis ball/cricket ball to work on the tissue at home as well.
2. Start stretching the pec major, minor and serratus anterior (all this front muscles that are tight).
3. Focus on scapula retracting exercises from various angles, also incorporate some external rotation exercises.
4. Keep you posture good as much as you can sitting standing walking etc etc.
5. Go get your whole posture checked out it may be that its not your shoulders that are the problem and just a symptom of a problem from further down your body.
Without pictures I couldnt give you more detail but if you search for shoulders on http://www.t-nation.com you should find a good article about rounded shoulders and other posture problems. There are shoulder health tests that diagnose what is wrong and are probably best performed by a physiotherapist or rehab specialist. Even if you have just one initial session to diagnose the problem and get a start on how to fix it you can usually work from there without a physio.