http://www.quora.com/How-do-stand-up-comedians-come-up-with-content-ideas-for-shows/answer/Tom-Shillue?srid=Hlr

An idea usually starts with having an experience that triggers an emotional response- often anger, frustration, bewilderment, or more rarely, happiness. Then comes the hard part- they set about figuring how to communicate this to an audience of strangers in a funny way.