Clothing should fit snuggly. Baggy and/or loose fitting clothing can catch fire more easily. Certain costumes that include capes, skirts and blouses should not be used as they can possibly get in the way of the fire props and catch fire. Unfinished edges with fray ignite easily, making even a seemingly safe pair of heavy jeans a hazard. Wetting down fray is a short term way to mitigate this risk.
Based on what prop you are spinning and how you intend to spin, this will vary. The main point is that you should be able to get caught in your prop, it’s imperative to practice with your prop in your costume before trying it on fire.