Reassemble the grill, then turn it on to its highest temperature setting and close the hood. There may even be a cleaning level on your burner dials. After a few minutes, open the lid and scrape the grates vigorously with a non-bristle metal grill brush. One I particularly like is the $15 Grill Bristleless Scraper from Taylor. This brush has multiple surfaces and edges to attack and remove grime from grill grates. It even has a handy bottle opener.
There are a ton of different grill cleaning gizmos to choose from, but nothing can beat a long handled wire brush, a wire bottle-style brush, a five-gallon bucket, and some elbow grease. Avoid toxic cleaning formulas. They can impart an off taste to foods, and aren’t necessary: warm water and dish soap and a paste of vinegar and baking soda are all the cleaning power you need. If your grill has stainless steel exterior surfaces, a good stainless steel cleaner (or vinegar) imparts an extra shine. You’ll also need some durable gloves and a few disposable sponges and rags. Pick an overcast day; hot sun can make it hard to remove streaks from cleaners (plus, it’s cooler working weather).
These are the workhorses of your cooker and because they come in direct contact with your food, they need to be clean. Start by removing the cool grates and wiping or brushing off as much of the big, loose debris as you can with a mesh or nylon scrub pad or a brush. In a sink or large bucket, mix up a sudsy solution of a grease-cutting dish liquid, like Dawn, and hot water and place the grates in to soak. If they don’t completely fit in, immerse one half, soak, then flip it over to get the other side. After soaking 15 to 30 minutes, put on some rubber gloves (like GH Seal holder, Playtex) and scrub the grates clean with a sturdy grill brush or scrubbing pad. Take extra care with porcelain grates, because you don't want to damage them.
Check the fuel lines for visible defects. If any fuel lines have unmistakable damage (like rips, holes, cracks), replace the parts immediately before your next use of the grill. Also be sure that any fuel lines have a smooth trail. You don’t want any of your fuel lines to be bent, because that will prevent the gas from flowing correctly. Check the exterior of the gas tank for any damage as well; things like dents, erosion, punctures, or any evident signs of damage. If you find areas that have obvious damage, you could potentially have a gas leak.