If you work on a computer, you will be spending long periods of time focusing on something at a fixed distance straight in front of you. To relax your eyes, regularly focus on something much further away (ideally, out of a window) and at things to your left and right. Also, roll your eyes from side to side.
Another soothing exercise is to cup your hands over your eyes so that no light whatsoever can get in for a few moments. Repeat these exercises throughout the day.