I use oven cleaner in a trash bag and let it sit in the sun for a bit. Rinse it off, and this will remove most of the oil and grease. Then scrape any remaining carbon off then wash with soap and water. Pretty simple. Any modern grate that needs more than this usually gets scrapped. However, sometimes the heavy gauge vintage stainless ones (and pricey 26.75/RK grates) are worth hitting with a wire cup brush on a grinder.