There are a few good ways of doing this. The last one I did for example:
A guy asked me for a dollar so I started looking around with a quizzical expression on my face. He tried getting my attention again then I looked down at my hands and began FREAKING. THE. FUCK. OUT. What I said was, "Oh no...Oh God No! Their back!! Not the voices!!! DEAR GOD NO!!!" then I ran away. That's more along the lines of convincing someone else that I am insane but still...the druggies that ask for cash around DC would have probably gone wandering around asking passers-by if they existed.
The other idea I have is to...
Look at someone then quickly double take. After the double-take stop dead and stare at that person, focusing anywhere but their eyes (or breasts if you are a male and they a female. Sexual harassment is bad [sub]apparently[/sub]). Let your mouth drop open slowly, raising a hand to your gaping maw and take a step or two backwards. Whisper to yourself along the lines of "of God" or, "Dear God" or, better yet, "Not again". Shaking your head is also a good move as you continue backing away in horror from the person who, at this point, is likely trying to get your attention to see what's wrong. This one should end in retching and/or mock tears as you turn around and run away.
This one I got from a book I think...I can't remember. All you do is hang out at an airport. Talk to someone and when they get up to leave for their flight you are supposed to say something along the lines of "Oh my...I wouldn't get on that flight. I just have a really bad feeling about it." You then smile and tell them to have a nice flight. Once again, that's more along the lines of them thinking you're crazy.