First, close your eyes. Shutting your eyes for a few minutes, or even several seconds, will refocus them and ease the strain. Second, blink a lot. Each blink soothes and moistens the eyes and eases tight eye muscles. Third, use artificial tears.
Eyes tend to get dry when they're strained and dryness can cause eyestrain.
Next to blinking, artificial tears are the easiest way to remoisten your eyes and ease the strain. If the computer screen is causing your strain, turn up the contrast on your monitor. The words and numbers on your screen are formed by fuzzy beams of light that are much harder to read than print on a page. To minimize the strain on your eyes, set the monitor contrast knob on high. Secondly, if you can, position your computer screen so that window light doesn't bounce off and create glare.