Hang a clip-on toilet bowl freshener over the rim, according to manufacturer instructions and restrictions. For example, some products have a chemical solid attached to the end of a plastic or metal hook. The solid is suspended on the inside of the bowl, touching the wall, on the underside of the rim. When anyone in the household flushes the toilet, the chemical solid is immersed in the rising water and showered by the water coming in from the tank into the bowl. The resulting solution helps to clean stains and might prevent black mold and mildew growth over the long term.