Upskirting: A Growing Trend

With the rise in camera phones a growing phenom has taken off with it: Upskirting.
Wikipedia calls it "a term referring to up skirt photography, images of the view up a woman's skirt (as seen from underneath), including shots of a woman's underwear or crotch, or exposing her vulva or buttocks." (The pic above is not upskirting, btw, but it's used to illustrate the point.)
It has received more attention lately as media outlets such as Huffington Post have even chimed in on the subject.
Now, although there is ta law against i but I don't think perpetrators realize it. And proving that you've just been upskirted? Forget about it. So, the question is, what would you do if you caught someone upskirting you?