I used to work in a fancy dress shop, designing looks and putting together outfits for parties and the occasional TV show. With such a vague brief there is plenty of room for personal interpretation. Clockwork Orange springs to mind, or, obviously, Alice in Wonderland.
HOWEVER, here is a short list of do's and dont's for fancy dress ( you may feel these are obvious but you'd be surprised how often these mistakes spoil parties):

1. Do not wear anything that covers your face: helmets, full mask or cumbersome hat. You will be too hot, unable to talk to anybody and unable to drink, eat or smoke.
2. Do not wear anything that extends more than 8 inches from your waist, tops. You will destroy your outfit, knock stuff over and you won't be able to sit down.
3. Do experiment with make-up, it's cheap and versatile. Once you've decided on your look, lay a tissue over your face and brush some powder over it. Do this several times on your lippy.
4. If you are using a synthetic material against your skin ( PVC, sateen, spandex etc) wear a light cotton undershirt and talc. You will sweat buckets, and this will spoil your outfit, including your hair and your makeup will run.
5. Decide on what type of person you are, if you are going to get tired of people asking you what your costume is then GO AS SOMETHING OBVIOUS.
6. Keep it simple, some of the best outfits I've seen for interpretative themes are based on plain black body ( t-shirt and trousers) with appropriate make-up ( psychedelic?) and accessories.