Rest your eyes on a distant object (if you're in a room, turn your view out the window, if you can). Focus on the object as clearly as possible while keeping the eyes and face relaxed. Take a deep breath, then gently slide your view to an object close to you.
Imagine your eyes seem to swallow the picture you see. Continue moving the view to a more varied distance while temporarily pausing. As an added bonus, if you see something really fun, smile, enjoy the beautiful scenery, and be grateful for your strong healthy eyes.