Heavy-duty grill cleaning is a once-a-year job best done at the start of each new grilling season. For gas grills, ensure all knobs are set to off and disconnect propane. When cleaning the main grill apparatus, focus on getting the outside shiny and appealing, but don't put too much effort into cleaning the inside; remove loose debris and ash but refrain from scrubbing away all that great cooked-on 'seasoning' that helps to control the grill's cooking heat, ultimately producing better food.
When it comes to cleaning grill grates, there are two schools of thought.
If you have loads of time on your hands and are into burning extra calories, go the cold-cleaning route. Use your sink, bathtub or a plastic tub large enough to accommodate your grill grates. Soak them for several hours in warm, soapy water. Then apply serious elbow grease to scrubbing, scraping, rinsing, and drying your grates and more effort to removing greasy particles and residues left behind in your sink or tub. Never use toxic cleaning solutions, rinse all solutions thoroughly and allow extra time for grates to heat on their first outing to ensure all residues burn off before cooking. A tip for cold cleaning without harsh chemicals is to soak grates in brewed coffee for an hour or longer before scrubbing.
Depending on your propensity to grill with sugary marinades, cheeses and other substances that ossify with intense heat over time, even the most assiduous cold cleaning may not produce pristine grates. If you are more interested in speed and efficiency, your mantra for cleaning and maintaining grill grates should be: heat and treat. If you are lucky enough to have a self-cleaning oven, simply stick your grates and other removable metal parts into the oven and run the self-cleaning cycle. The extreme high heat will burn away grill grime and muck, leaving you with clean grates and a clean oven for the summer.
If you do not have a self-cleaning oven, invest in a high-quality wire grill brush and pair of high-heat-safe gloves or mitts. When the grill is hot, brush the grates firmly with as much pressure as you can apply safely without toppling the grill. After grilling, either brush grates while still hot and, when cooled, treat lightly with cooking spray, vegetable or olive oil to protect; or, simply leave the grates dirty as protection against the elements until the next time you grill. If you do not have a grill brush, a scrunched up piece of heavy-duty aluminum foil or half an onion used with extreme care (and high-heat-safe gloves) will also work to remove most stuck-on grime when the grill is hot.
For charcoal grills, be sure to remove cooled, spent ash after every use (and especially before it gets rained on). Gas grill briquettes should be periodically removed, shaken or brushed and gas jets cleared of grease and food debris, which can prohibit even distribution of heat. Flipping gas-grill briquettes periodically takes advantage of the grill's high interior heat to clean them with virtually zero effort.
Finally, when not in use, remember to always cover your grill. Good eating!