I am not clever enough to write this. Copied from someone else who posted without attribution. If you know who wrote it I'll happily give credit.
As a cyclist, you are much, much better equipped for Halloween than the average person. Why? Because you already wear outrageous costumes on a daily basis. Think about it. Even though you are a (presumably) sane adult, you wear a shirt that would look much more at home on a superhero. You wear shorts that are much, much too tight, as if you were on your way to lead a jazzercize class. You wear a hat that belongs on an alien. And, to top the whole look off, you wear what sound and look like tap-dancing shoes...
By spending just a few extra minutes, you can alter your cycling outfit for the evening, making it so you're not just "a cyclist" at the party, but a very particular sort of cyclist. Simply follow these easy instructions.
Doping Cyclist: Dress up in full pro kit. Use a marker to draw needle tracks up and down one arm. Tie a length of surgical tubing above one elbow and leave a syringe sticking out of your vein. Wheel around an IV tower for the duration of the party. Stuff your jersey pockets with bottles of drugs. When anyone asks what / who you are, respond that you are a professional cyclist. When they ask what all the needles and drugs are for, say you have no idea what they're talking about. No matter what, do not admit you have any drug-related items on hand.
Mountain Biker (If You're normally a Roadie): Prepare for the party by gaining 10 lbs and getting 20-30 tattoos. Wear baggy pants -- baggy enough that they keep falling down. Arrive drunk at the party and continue to drink once you get there. Insist you have mad skillz.
Roadie (If You're normally a Mountain Biker): Prepare for the party by putting a stick in your butt. Wear uncomfortably tight cycling clothes for the party, drink nothing but thrice-filtered water and tell everyone exactly how many calories and fat everything they're eating contains.
Triathlete: Don't come to the party, because you've only done four workouts today and still need to get in one more and you don't want to break training, no matter what.
Recumbent Rider: Tape your glasses together, somehow manage to affix a pocket protector to your jersey, and loudly and insistently explain to all and sundry that recumbents are really much more comfortable and practical than "wedge" bikes. Talk a lot about prostates.
Recreational Cyclist: Wear street clothes with your right pant leg cuff completely greasy and shredded. Wear a bike helmet backwards and *censored*ed to one side. Tell people that you're starting to bike again to get back in shape.
Commuter: Wear street clothes, but carry a backpack or messenger bag full of what are clearly stinky bike clothes the entire evening. Make your helmet hair extremely obvious. Keep looking for opportunities to casually introduce the fact that you are a bike commuter into every single conversation in which you participate.
Fixie / Track Cyclist: Dress the same as a road cyclist, but you must always either keep walking or -- if you must stay in one place -- you must trackstand by moving a couple inches forward, then back, then forward and back again.
See? Easy, effective, and totally transformational. Nobody will recognize you. Better start preparing that "Best Costume" acceptance speech now.