If you want the 2000s look: go for a furry animal set. The hood, tail and fluffies are what make it read as Y2K. Add platform boots and glitter and it is done.
If you want to wear it again: build from a corset. A corset with ears or wings is a costume in October and a going out top the rest of the year. A one-piece novelty rarely leaves the closet again.
If you are between sizes: size by bust and waist, then size up. The corset or bodysuit is the fitted piece in most sets, and a lace-up back lets you bring a larger size in. It cannot let a small one out.
If you need extended sizes: check the size list on each listing. Plenty of the costumes and corsets run past XL, and the size chart on each product page gives the measurements.
If the party is dark: pick fur, satin or holographic. Texture and shine pick up phone flash and club lights. Flat matte fabric disappears in both.
If it is a couples or group costume: choose a family, not a character. Animals, devil and angel, or a set of uniforms all read as a group even when every outfit is different.
If you have a date in mind: order early. Popular sizes sell through in October, and the finishing pieces (ears, hosiery, pasties) are easy to forget until the day of the party.