Start the season with clean grates, both for sanitary reasons and because you want to kick off your grilling with a beautiful clean grill and beautiful clean grill marks on your food. For a gas grill, turn all the burners to high, shut the lid and let the grill heat for 15 minutes. Open the lid and hopefully everything stuck to the grill will have burned off. Then, just give it a good scrub with a grill brush or grill scraper. Make sure no bristles get stuck on the grill rack. A wadded-up piece of foil held with tongs also does a good job. You can give the clean grate a light brush with oil while it awaits your next grilling session.
Haas advises, "If it's been awhile since you've cleaned your grill grates, remove them and take a nylon sponge or hard bristle brush to them along with some tough cleanser. Make sure to rinse and dry them thoroughly before placing back on the grill."