You a boy or a girl? What genre do you like?
Edit:
Princess Leia if you're into Star Wars
Lydia Deetz from Beetlejuics if you're more into wearing a lot of black
Mary Poppins
The little girl from The Exorcist (you could get yourself a white nightgown and put it on backwards like when her head spun around with a lot of creepy makeup)
Carrie (put on a light pink prom dress and then cover yourself in fake blood)
One of Charlie's Angels (the 70s version, not the newer)
Holly from Breakfast at Tiffany's (that would be alot of fun--pretty dress, lots of jewelry, great updo)
Annie Hall
Barbarella
Marge Simpson
Beatrix Kiddo from Kill Bill
Mia Wallace from Pulp Fiction
Pippi Longstocking
Viktor Hempel from Victor Victoria (a really fun one--the movie starring Julie Andrews is about a woman who dresses up like a guy who dresses up like a woman)
Buffy the Vampire Slayer
Morticia Addams
Cherry Darling from Planet Terror