When you first sit down to read, take some long, slow deep breaths. Sit up so don’t compress your lungs. Deep breathing relaxes eye muscles and re-oxygenates the blood.
Position yourself so you don’t look at a wall but down a room or out a window, look up occasionally and let your eyes wander around, focusing at a distance.
Get up every 20 or 30 minutes and stretch or walk around. Do something else that will give your eyes a minute of rest. Stretching the whole body helps release tension and improves circulation to the eyes.
Change the position of your reading material periodically. This challenges the focusing muscle inside your eye.