Male of female?
Male - go as John Lennon, just a loose linin shirt and round sunnies will do it, especially if you've got messy or long hair. Doesn't matter if you don't look like him at all! Failing that, just draw/buy a mostach and call yourself Freddy Mercury. However, if you have chinos, polo shirts and sweaters, just go as sebastian from brideshead. all you need is a foppish fringe and a teddy bear.
Female - hmm, depending on the female, i'd say wear a red/sexy/vintage type dress and say you were a sex kitten orgo as a popstar from the 70/80's you admired. depending on how much flesh you wanna show, you could be a bikini model, or if you wanna cover up, a sixties beatnik (stripy shirt, jeans and boots. converse will do it)
This is what I do for a saturday job! I work in a fancy dress shop, so a physical description and an idea will help a lot!
or ignore me, either way.