Tap your foot. Your skinnier friends are probably fidgeters, who burn up to 350 calories a day just by tapping their feet or being restless. Try it for a few days. Walk around while you're on the phone, or tap out a tune with your hands or feet (in the privacy of your own office, of course).
Seems simple enough, right? Plus, it'll give you a good excuse to dance a little at your desk to any new tunes you acquired this season!