Active tension drills to build shortened end-range mobility in the shoulders, targeting especially the trapezius muscles!
These shoulder mobility drills add both strength and flexibility to your shoulder girdle (scapula). You will be surprised how tough the prone exercises below can be.
Although these drills can be labeled as "mobility drills", you can expect to build some serious strength and size to many of your scapula muscles. This is because mobility is strength.
All of these exercises will mainly work the muscles in their shortened lenghts in the shoulder joint's end-range of motion (active tension). This basically means that you will actively increase the range of motion of the shoulder joint and build strength there.
Read this article to understand different kinds of mobility. An excerpt below:
The difference between active flexibility and active tension:
1. In active flexibility you first stretch the muscle (lengthen it) and then contract the muscle. Example: overhead tricep extension where you are focusing on the initial range of motion where the arm is bent and the triceps are stretched.
2. In active tension you focus on building strength in the shortened state. Example: tricep kickback when the arm is straightened.
Who should do these exercises and why? Anyone who wants to improve their strength, range of motion of their shoulders, prevent injuries, maximize performance and improve their posture (for example, this can be excellent for the hunched over posture).
For a structured mobility program, see Movement 20XX. M-20XX consists of many different types of mobility training and has the most well-rounded approach to develop good structural balance in the entire body.
Prone Active Tension Drills
Prone Overhead Raise

In the prone overhead raise you move your arms from the sides to your overhead while your stomach is facing down.
When your arms are pointing to the sides, you are activating more of your posterior deltoids and middle traps. When your arms are pointing directly forward (overhead), you will be utilizing more of your upper traps.
You move the arms up and down while you are keeping tension in the muscles and contracting the muscles in their shortened states. You can make the exercise harder by using small plates.
The key is to try to find tight spots and focus more on them to build strength in these weak areas.
Prone Overhead Pulse

In prone overhead pulses you aren't merely moving the arms overhead in the end-range of motion, but you are also doing small up-and-down "pulses".
Do small pulses in every spot and direction imaginable. This will maximize the results and can produce great gains in your shoulder mobility.
Although you are doing pulses, you should still keep constant tension in the muscles. The pulses need to be "active" and controlled.
Prone Press

In the prone press, you are using a stick to press overhead while you are prone.
Using a stick is a great tool because it will fix your arms in different widths which would otherwise be very hard to do.
You can do the prone press with different arm widths: narrow, shoulder width and wide. You will mainly work the traps but also the shoulder muscles. You can make the exercise significantly harder by adding small weight plates in the middle of the stick.
It's also possible to do overhead pulses with the stick. This will allow you to do overhead pulses with different grip widths and in different arm lengths (how much your arms are extended forward).
Shoulder Dislocations (Standing and Prone)

You can do shoulder dislocations with a stick standing up, in a bent-over position and prone.
You can also vary the width: the narrower the width, the harder it will be for your shoulder flexibility. Your hand placement (overhand or underhand) will significantly change the nature of the exercise due to the shoulder rotation (=do both).
When you are doing shoulder dislocations upright, you will mostly target the shoulder muscles. When you do dislocations prone, you will emphasize more the trapezius muscles. By far, the prone variations seem to be the hardest and also the most effective.
What makes the shoulder dislocations such an effective exercise is the fact that holding onto the stick will force your shoulders undergo range of motion you naturally would not be able to do.
Similar to doing any mobility exercise effectively, you need to focus on slow and controlled repetitions. The common mistake people make is to do the shoulder dislocation too fast - this doesn't produce outstanding results.
Try all of these exercises in your mobility workout, or do them before or after a workout for a couple of sets until you feel your muscles get tired.
Train hard stay safe.
How often or how many times a week should I do these exercises? I currently do them after every gym workout, and I go to the gym 3-5 times a week.
After workout is great but you can also devote an entire workout to just shoulder mobility like this. 1-3 times per week should be enough.