I have a Hallowe'en/Samhain party every year and offer a themee to my friends and invitees. (It is known as the Gothic Samhain Soiree, so they can always just wear black and feel like they have made an effort).

This year's theme: Denizens of the Otherworld.

Past themes: Favorite Dead Fictional Character; Your Evil Twin

I have dressed as The Madwoman in the Attic, Jane Eyre, and a vampire, and oh I can't think what else...this year, I think I shall draw vines and leaves on my face and wear ragged green clothes and go as a tree sprite or some such thing...

Try hitting a thrift shop or vintage clothing store; they tend to really play up their Hallowee'en usefulness at this time of year...