Miss America: Wear a cocktail dress (or a swimsuit if you're really brave) and then make a sash (perhaps out of toilet paper) that says Miss (fill in the blank).
Nerd: Wear an outfit of high-waisted pants, a plaid shirt and a cardigan. Grease your hair. Tape your glasses. Add a pocket protector and some pens.
Spy: Wear a trench coat, sunglasses and a fedora if you have one.
Pirate: Wear a bandanna on your head, one big gold hoop earring, a billowy white shirt and rolled-up black jeans. Make a sword out of cardboard and fit it into your belt. Pin on a skull and crossbones.
Ripped from the headlines
Hockey Mom: Wear a hockey jersey and carry a baby doll.
Stock Market : Wear all black and then use masking tape to stick erratic zigzags onto your body, pointing down.
Exit Poll : Tape questions and check marks onto your back. On the front you can pin an "I Voted" pin.
Joe the Plumber : Wear jeans or work pants and any sort of shirt and maybe a trucker hat. Carry a plunger and make sure the jeans can ride down low. (You could also do Joe Six-Pack with the jeans, a plaid shirt or a sports sweat shirt and then pin plastic six-pack rings all over yourself.)
Blog : Wear whatever you want and write the words "blah-blah-blah" on lots of pieces of paper and pin them all over yourself.
The Biggest Loser : Dress in something four or five sizes too big.
Conceptual
Chick Magnet : Hot-glue marshmallow Peeps candy onto your outfit. The hard part there is finding Peeps this time of year - though it's possible to download pictures and tape those on. Von Marko also suggests using pictures of chicks from a farm catalog.
Chip on Your Shoulder : Dress in regular clothes and glue a poker chip to your shoulder.
Identity Crisis : Put "Hello my name is ... " tags with random names all over your body.
Shot in the Dark: Wear all black and hang a shot glass around your neck.
Past Your Prime : Put a sign on your shirt that says, "Sell by Nov. 31, 2001" (or any date that has already passed).
Black Mail : Wear all black, and attach a postage stamp to your chest.
Head of Lettuce : Attach real or fake lettuce leaves to a stocking hat.
God's Gift to Women : Find a box large enough to fit around your body. Cut some holes for your arms and head, and then cover the box with wrapping paper. Add a large bow. Attach a tag that says "TO: Women, FROM: God."
