How to improve poor flexibility in right shoulder?

I've just noticed I have poor flexibility in my right shoulder. It seems to make sense that I could drop some time just by improving this. Does anyone know any good stretches or exercises I can do to address this? The flexibility test I did was to try to touch fingers of opposite hands behind my back by reaching one hand back over shoulder and the other hand just reaching up from behind the back. It's a test I learned of recently when I became a personal trainer. I am wondering also if years of swimming could actually have caused this as well.