The most common form of grill-top smoking is with hardwoods chips. They come in about every species of wood you could conceivably want to smoke with, from pecan to apple, mesquite, hickory, and more. When in doubt, you can never go wrong with hickory. Use the chips dry to impart just a tinge of smokiness as a finishing touch. Or for deeper flavor, soak the chips for a half hour and spread them directly on well-lit briquettes or arranged in a smoking box, or create a little boat of tinfoil to rest above the flames of a gas grill. Soaked chips will simmer for a half hour or more, plenty long enough to dress up burgers, steaks, or chops.