Looking at a monitor for extended periods of time strains the focusing muscle in your eye. Your eyes will also dry out very quickly, especially if you wear contacts. Use a timer like this one:
click here to remind you to look away from the computer and re-focus your eyes. I set it for every 15min if I'm doing 5+ hours straight on the computer or if I'm editing video. I sometimes go 2+ minutes without blinking when editing. A brief walk out of the room also helps. You will be amazed how much better you feel after trying this for a full day.