This is what my best friend Rebecca is going to be this year, I hope it works.
Dress: First get a ballroom type dress (maybe like the kind belle or sleeping beauty wears).
Accessories: White silk (or fake silk will do) gloves, jewelry (optional).
Shoes: If your dress will cover your feet you can were sneaker or flip flops, but if it covers your feet stick with comfortable shoes or high heels (I wouldn't do high heels in less you must or just going to a Halloween dance and not trick or treating).
Make up: You might want to use fake blood or not its up to you. Also with facial make up make your self look as dead as possible (you know a lot of greys and whites) also you can get grey spray on spray on hair dye for a few streaks.
You might want to get the stuff from Goodwill or some place you can get inexpensive stuff that you don't mind messing up.
Instructions: First make a few rips and cuts on dress add fake blood if you want to. Second do all your make up. Third, put jewelry and gloves on and basically that's it, hope you like it.
Thanks to Siobhan for this scary idea.