Do it right: Take the hands straight out in front of you and parallel, palms facing each other. Twist the wrists inward so that the palms face out and then cross the right arm over the left, bringing the palms together. If this hurts your shoulders or wrists, simply do the move without the twist. Round the back, reaching the arms away from you as you relax the head. Contract the abs and imagine that your stretching up and over an imaginary ball rather than collapsing in the middle. Hold the stretch for 30 seconds and repeat.
For more stretches you can do at work, check out my Seated Stretch for the Back, Shoulders and Neck.