I've let mine sit in a pail of hot water with either laundry detergent or Mr. Clean. Whatever, you want something that can cut through grease so Dawn dish soap is also a good choice. If they look like they have oil or grease stains, a scrub of the area with paint thinner (not lacquer thinner) beforehand helps break it down. I leave them soak for a while in hot water then scrub them good with a dollar store scrub brush. Once "washed" I give them a good rinse, leaving them sit in cold rinse water for a while and sometimes rinsing twice. I then hang them to dry outdoors on a warm sunny day. I don't use or do anything that might harm the integrity of the belt fabric.