act startled and say "oh my! i almost forgot! let me get my costume" and reach into your bag and pretend to dig for your costume. if you do not have a bag or purse, a pocket will suffice although the effect won't be as dramatic. if you do not have a bag or pocket, you're an idiot. do people really walk around pocketless and bagless? who are these people? how do you carry stuff?
anyways, feign diligence for your search and say things like "i can't believe i forgot, thanks for reminding me" and "oh, almost got it..." and "not quite...there, oh no, there".
finally, exclaim "got it!" at which point you remove your hand from your bag or pocket and smack the costumed person upside the head.
if you don't want to go for the violent approach, you can be like Ricky and pull out your middle finger for the big FUCK YOU!
That's fucking genius. I love that idea and your site kicks ass too. I just hate stupid people in general so that's something I would definetly do, cause I'm just a sick bitch with a twisted sense of humor.