Of course, the most obvious and famous Spears' costume is a solid option, since life would clearly not be the same without 1999's "Baby One More Time" video.
What to wear: A white shirt with buttons (note: you will not be using the buttons), tied up in a knot. A short plaid skirt, gray cardigan, knee socks, and Mary Jane heels. Braid your hair into pigtails and add marabou ponytail holders.