This is something I have trouble with as well, moreso noticed once I started doing overhead squats, weighted pullups and turkish getups.
I went and saw a physio about 3 times, he gave it a bit of a massage each time and also advised on some exercises to do. One was to stand arms length from a doorway, hold onto the doorway and twist whole torso til you can feel it in your pec. May have to adjust the height you hold the door at to suit you.
But the thing that helped me the most was just lying on a lacrosse ball. Lie down on your back, with a lacrosse ball under your shoulder blade and and spend like 5 minutes going very slowly with that arm moving through full rom in all directions with shoulder as the pivot. Then put it under the other shoulder blade and repeat with the other arm.
^ This is good advice.
With the pec stretch, you can also just lean through the doorway while hanging onto the frame. That way you can use your weight to get a better stretch. Also, the pec is a fan shaped muscle, so to stretch along all its lines of pull, you should vary your arm height from horizontal to 45 degrees above horizontal.
With the ball massage, I also recommend sandwiching the ball between your back and a wall and rolling it around. If you find any tender spots, roll over them a few times. For better ball control, put the ball in a footy sock and use the sock to stop the ball from getting away from you.
Also try this exercise. Stand with your back to a wall with your butt, shoulders and head pressed against the wall. Tilt your pelvis backwards so that your lower back is flat against the wall. Now raise your arms to the side so that your upper arms are horizontal and pressed against the wall. Bend your elbows to 90 degrees so that your forearms are pointing upwards. Rotate your hands so that the backs of your hands are facing the wall. Try to press your forearms, wrists and hands against the wall while keeping your back and shoulders flat against the wall.
Next, slowly bring your hands to the top of your head while keeping your arms touching the wall and your lower back flat against the wall. Then slowly raise your arms so that they are 45 degrees above the horizontal with elbows extended. Lastly, slowly raise your arms overhead until your arms are touching your ears. Elbows should be extended and your arms should be in contact with the wall at all times. Same for your lower back.
If you are as tight as I am, you'll look like a mongoloid the first few times you do this. I still can't get my arms and hands flat against the wall in all positions, but my flexibility is slowly improving.
Hope this helps.