I try not to use my Ren Faire garb for Halloween, though sometimes I'll used bits and pieces of it. (A bodice or a cloak can come in handy for lots of costumes.) Any time I get something for a costume, it stays in my costume closet to be reused in other parts. I have a poodle skirt, a toga, a red cape, etc. It's fun to pull stuff together sometimes.
Last year I was Athena. Toga, plastic Roman war helmet (couldn't find a Greek one), a toy spear, and a little stuffed owl on my shoulder.