One of the most common causes of eyestrain is a computer screen, however any kind of continuous concentration can cause the same symptoms.
Blink!
Staring at a screen can shut down the blinking mechanism, allowing the surface of your eyes to dry out and become irritated.
Concentrate more often when you have to do close work.
Stare away
Every once in a while, look across the room allowing your eyes a chance to refocus and rest.
Use an air-screen
As the fan inside your computer blows to cool the computer's inner workings, it also blows a steady stream of air - drying air - toward your eyes.
Tape a piece of cardboard to the top of your monitor so it baffles the air away from your eyes and towards the ceiling.
Rethink lighting
If your eyestrain is worse after watching a bright screen, change the lighting around your work area.
The background light level should be low and offer contrast.
You should not see any bright sources of direct or reflected light in the screen.
Use a glare screen
Add a screen to minimize the amount of glare reflected back into your eyes.
Wipe off dust
Your eyes have to work harder when they are trying to see past dirt, smears and fingerprints.
Clean your computer screen with a damp or anti-static cloth.