After every 20 minutes of looking into the computer screen, turn your head and try to look at any object placed at least 20 feet away. This changes the focal length of your eyes, a must-do for the tired eyes.
Try and blink your eyes for 20 times in succession, to moisten them.
Time permitting of course, one should walk 20 paces after every 20 minutes of sitting in one particular posture. Helps blood circulation for the entire body.