1) Learn to cast a shadow over your face regardless of where the light source is situated in the room.
2) Show more skin. When heroines turn evil they invariably get shorter skirts and plunging necklines.
3) Accessorize with leather and chains for that badass look.
4) Ragged edges show how you have symbolically trashed your "goodie" costume, also that you are just so bad that you can't be bothered to hem.
5) Evil hair is usually bigger than good hair. Get in lots of gel for that snakey look.
6) Colours: Black, obviously, but doing a version of your regular costume in darker shades also works.