Hello,
Lots and lots can be done to help with sub-luxing (partial dislocation) shoulders - for starters have a look at http://www.sportsinjuryclinic.net/cybertherapist/front/shoulder/dislocated_shoulder.htm and http://www.sportsinjuryclinic.net/cybertherapist/front/shoulder/shoulder_instability.html which both have quite a lot of information on the anatomy of the shoudler joint and what causes sub luxations.
You can view here http://www.sportsinjuryclinic.net/cybertherapist/front/shoulder/dislocated_shoulder/dislocated_shoulder_strengthening.php information on strengthening the shoulder, however I would strongly suggest you visit your GP and get referred to a physio (or see one privately if you can) to get working on a rehab plan.
Ruth Cheesley BSc(Hons) MSc MCSP
Chartered Physiotherapist