Develop your pectoral muscles if you haven't already. Do strengthening exercises like push-ups, dumbbell press, chest dips and butterflies. As you get stronger, increase the amount of weight or resistance you use for quickest results.
Practice flexing your pecs in quick, jerky motions by clasping your hands together in front of your torso and pressing your palms together. You should be able to feel the muscles tense and, if you look down, observe them "dancing" in staccato movements.
Once you have a feel for the muscle contraction, practice moving each pec independently without your hands clasped. Although you can do this just by flexing the muscle, it helps to give a tiny jerk of the shoulder down and forward.
Having mastered independent pectoral flexing, begin choreographing a routine that involves flexes of the right pec, the left pec, and both pecs at once. Use a mirror to assess yourself. For added flair, consider interspersing other kinds of flexes -- bicep, deltoid and others. Set the routine to music (optional).
Find an audience and perform the routine, or simply improvise a few pectoral movements when the right moment arises.