I'll start this:
Shoulder "dislocations" - great for improving shoulder mobility
Anterior shoulder stretches:
Posterior shoulder stretches:
Remember that you can actually injure yourself while stretching if you stretch "wrong". Stretches should be done slowly, consistently, and repetitively. They should not be done quickly or as hard and fast as possible.
Shoulder injury is often caused by weakness in or overloading of the rotary cuff. It's badly developed by most people since they don't really train it. Badly executed Bench Presses seem to be a major cause too, by putting too much strain on the shoulders.
So in this light I think it's important to train the muscles involved with your rotary cuffs.
There are a few very easy to execute exercises that can help in strengthening your cuffs, with only a dumbbell:
Of course you can use cables in stead of the dumbbells if you happen to have those.
EDIT: BUT! If you ever feel a strain on your shoulders or pain while exercising, go see a kinesiologist!
When doing bench press, keep your elbows close to your trunk (no more than 45 degrees out) and bring the bar to your sternum and don't let it creep up closer to your head.
I was taught how to bench press wrong in school (elbows out almost 90 degrees, bar higher, flat back), and it wrecked my shoulders for years before I realized what was wrong and corrected it.
I'm not sure if you are referring to how to avoid having more dislocations after the initial one, or if you mean how to strengthen the shoulders to decrease the risk of shoulder dislocations.
If it's the former, as a person who has experienced multiple shoulder dislocations, here's my most important advice: see a physio! There is no way to replace a physiotherapist's expertise and advice on a Q&A like this.
If you mean the second (decreasing the risk for potential dislocations) I would say unless you are genetically predisposed to get this kind of injury or do some exercise where the shoulder joints are put under serious stress one should not be too worried.
I will however try and elaborate on what exercises I have come across with as an aid for recovering from shoulder dislocation. There are two main types of exercises that are recommended in this case; stability and strength exercises.
Stability exercises are usually static, you are supposed to hold a particular stance for 30–45 secs, e.g. hold yourself in the initial position of a push up, hands on the floor, or on a vibration plate/medicine ball etc. Another version is vibrating a "gymblade" or tugging on a rubber band, rather small movements but significantly challenging to keep up for a while. These exercises target the smaller muscles and the teamplay of the muscle groups.
Strength exercises are typically aimed at larger target muscles, and aim to get them stronger so that your shoulder have more resistance to risky movements. Some of the most typical exercises are in/out rotation on the left/right axis and on the up/down axis. These can be done sitting or standing. Diagonal pulls are also an alternative once the regular rotations are less challenging.
Also exercises for auxiliary muscle groups such as biceps, triceps, pecks and upper back muscles (particularly trapezoid) help to some extent.
There are a few answers here for shoulder injuries, you can search "shoulder" to get a lot of good information. Here are a few.