Don't take direct suggestions (example "Your sig should be 'dah dah dah dah doo'"). Just wait until you hear or read something memorable. I also kind of really despise sigs that acknowledge that it's a sig, like, "I can't think of a good sig right now so I will add this explanation as to why I don't have one," or "[sig not found]." Yes, I just wait until I hear or read something funny.