Have a medical eye exam including a refraction (glasses test) to see if you have any disease or need glasses. Every 15 minutes look away from your screen, across the room, ideally out the window and blink 10--15 firm blinks. Find a good artificial tear that works well (e.g. Blink, Systane) and use prior to starting your work day and every 3-4 hours, don't have air blowing on you form heat/AC vent.