Yoga has completely changed my riding it's amazing! if you don't want to join a class there's some great videos on YouTube for yoga for shoulders
Sit up straight as if you have a string pulling on your head, push out your chest alittle bit because this naturally opens up your shoulders.
Sit back. I always like to think I'm laying down on the horse while I'm riding. Also just be conscious about your shoulders when you ride. Sitting with your back straight can also help.
If u have a lot of trouble smartpak sells a shoulder and back thing that teaches your shoulders to stay back. After a while you won't need to use it anymore and it will be natural for you!
Ride from your elbows. Get contact and put a bit of weight into dropping your elbows straight down then you won't have a tendency to grip with hands and forearms, causing tightness in the front of your shoulders, hence pulling your shoulders forward. This will cause you to have to develop more core muscles. I'm a coach and I see this quite a bit