This is more directed to adult dresser-uppers!
Similar to your PJ's as a costume tip, I would suggest a multi-purpose costume.
These days, costumes $$$ are OUTRAGEOUS!!! And to buy a new one every year is just silly and certainly not cost effective. I mean, for the little ones, it's fun to dress them up in brand new too-cute costumes every year, b/c they're only that little for not very long, so that's different.
But as an adult, I find it useless to buy these expensive costumes that you'll never use again, so i try to wear something I own and make it a costume for very cheap.
For example, I was at the mall, and spotted this adorable Red dress with white polka dots and thought it was really cute. So, upon buying it, I thought I could wear it to work or out at night and PLUS I decided then and there that I was going to Minnie Mouse for Halloween this year b/c it is SUCH a Minnie Mouse Dress!