Josie and the Pussycats:
Works best with a trio, but you can of course do just two or choose one that you look most like.
A really cute shirt and either tight pants, short shorts, or a mini skirt; heels or sandals; cat ears (you can find them at almost any store that sells clothes).
Josie: short, brown, straight hair flipped out at the front (works best w/layered hair.
Melanie: blonde hair, shoulder length at least, curled in those big fat curlers.
Valerie: black hair, mid length,poofed tiny curls (you'd know what i mean if you've seen the movie.
To make the costume even more awesome, go to a toy store and get a fake guitar (for Josie and Valerie) and a pair of drumsticks(for Melanie) also get a headset you can go to any store that has electronic stuff and get a headset for a phone (they only cost around maybe $5 - $10)
Thanks to ECM for this great idea.