Here's another method you can practice right at your desk. Progressive muscle relaxation involves sequentially tightening and relaxing muscle groups; over time, this practice can help you become familiar with the physical signals of stress in time to head them off.
Most practitioners start with the feet and move upwards on the body, ending with the face. To start, slowly clench the muscles of your right foot, hold for a count of ten, then slowly release. Repeat on the left side, moving to your calves, then thighs, abdomen, then the hand and arm muscles, the back, neck, and finally the head and face.