|Photo on right of me as Beanie Baby by Emily, with bonus Popple ears(?)|
Part of throwing a good party is having guests embrace the theme. When you see a room full of people dressed as forgotten pop culture, movie stars, or 1920s guys and dolls it tells at least as much of the story as food, music, or decor does. In a way people are the greatest decor. Not to objectify you all, but basically I see you as objects. Seriously though, you can't have a party without any guests, and if the guests are on theme then the party is.
So as a guest, do what you can. Plan ahead, find something fun for you to wear that goes along at least a bit, maybe borrow something. I also keep a stash of costume parts (including cat ears and a tail, which I used to build my Beanie Baby costume)
As a host, my friend and frequent co-host Phil and I have developed a system of having extra costume available, from a stash of boas and hats to a mask making station at the party.
My friend Emily the ever-prepared grows her hair up to a year in advance for costumes. I once shaved half my head to be Delirium from Sandman. Please feel free to share and suggest awesome costume things you have done.