Take one of those brittle plastic cups. Put it under your armpit when no one is looking. When everyone is looking, put your hands on either side of your head and pretend to twist quickly, simultaneously crushing the cup stored covertly beneath your arm. The cup will make a hideous cracking sound. Sigh as if you had just received an hour-long massage from a licensed physical therapist. People around you will gasp and recoil in horror, unless they too have read Penn and Teller's excellent book How to Play In Traffic, from whence I discovered this ridiculously simple but remarkably effective trick.
Jillette, Penn, and Teller. How to Play In Traffic. New York: Boulevard, 1997.
machfive pointed out that the trick existed long before Penn and Teller included it in their book. Apparently their radical ideas about breaking one's own neck had already occurred to others. Nevertheless, the book is as good a reference as any for this and other tricks, and probably the most well-known one pertaining to this stunt in particular.