Get the grates hot, so the coals are burning white. Use a wire brush to scrape off any excess fat and grease. If you don't want to use a wire brush, spray some lemon or orange onto a cloth. Use tongs to hold the cloth and wipe the hot grates clean.

Another solution is boiling soapy water in a large basin and scrubbing the grates with a brush. Wear protective gloves and not get any hot water on the skin.