Forget dislocations people don't control them properly and tend to lead to poor movement. Use a power rack and a bar. Set it around the height of the bottom of your sternum and then stand in front of it with your back to the bar. Grab the bar over the top at the widest point you can.Straighten your arms, dont push your chest out stay tall and in good posture and squeeze the bejesus out of your back muscles to get the shoulder blades down and back. This should create a stretch in your pec minor and pec major. It can be made harder by bringing your hands closer together or putting the bar up higher or bending your elbows. Make sure your shoulder never rolls forward at the top. Search mobility wod for the shoulder extension stretch it is near the start in the first few pages. This is a better, more controlled, easily progressed stretch that can get you a more pliable shoulder that allows you to retract your scapula properly and hold it when in deep extension (bottom of bench). It is hard to describe but easy to do once you see it.
There are a few good ones of there for internal rotation. You should also look at getting a hard ball into the pec minor to release it before the stretch. Do movements such as deep extension at the shoulder to contract and relax with pressure being applied from the ball. Hold the stretch for at least 3 minutes working on relaxing into it.
Work on light full retraction pushups, DB bench press etc as a means of actively stretching the muscles under load and teaching yourself good position. All of this together works quite well. Andy makes a good point but make sure you're not already injured first and that you start very light until the flexibility returns.
There can also be other issues such as restrictions in your scapula muscles (rhomboids, traps, rotation cuff) that can reduce your ability to move properly through the scapula. Your clavicular head of the pec major (upper) can also be very tight and restricting your ability to move the shoulder too. It all depends on what you need.
