Like other muscles in your body (legs, arms and back), the eye muscles should be stretched regularly. Stretching keeps muscles loose, strong and flexible.
Eye muscles can be stretched simply by moving them slowly in their extreme periphery of motion. Begin by holding an object (such as a cup) slightly above and in front of your head . While keeping your head still, slowly move the cup in a large circular arc just within range of vision. Following the cup with your eyes without moving your head. This will force your eyes out to their furthest outer range of motion. Slowly arc the cup around to the right then around to the left follow.
We tend to look down far more often than up (while eating, reading, etc....) therefore I tend emphasize upward stretches.