My thought would be to try to find a "Cinderella" style prom dress (strapless, full length, puffy ) and a tiara.
there ya go: a princess
Or if you really wanted to be modern you could get a dress with no straps and a short length. I can't think of a better way to describe this than finding the most "modern looking" one, showing as much skin as possible while still being modest enough to run around in on Halloween. Plus a tiara .
Too little time to try to find pictures. Sorry.
-Then again, both of my ideas require 80 dollar dresses.... -