Whether you are a top tier athlete, or just a weekend warrior, short-duration stretches are not adequate for injury prevention. In sports like baseball and tennis, one of the most common types of injuries are shoulder injuries, due to the stress caused by swinging a racket or bat, or throwing a ball with all your might.
To keep muscles loose and elastic, I developed this stretch based on the Tai Chi forms "Close Tai Chi" and "Return to the Mountains" (see the video).
I know you don't have a wall like the Green Monster to stretch with, but any smooth, tall wall will do! Happy Stretching!