LazarusRevives - this is great advice and something I have experienced as well. I used to bench on a bench so narrow my shoulders were in free fall when I pushed up, and they would pop out to an extent. very bad in the long run. avoid that by thinking explicitly about your shoulders while you lift. (or just find a different upper body exercise that isn't benching, if you go to a normal gym this is easy.)
some stretches you can try -- one, do arm rotations before every time you use your shoulder muscles; arms straight out sideways, doing concise little circles and then widening them. two, get an elastic band, step on one end, and bring the other end out away from your body with resistance.