The first thing you need to do is take all of the interior parts, such as grill grates, deflector plates, heat plates, etc. out of your grill. Check all parts for rust and holes and replace parts that are too worn to last another year.
Take all of your grill's interior parts and soak them in a mixture of warm water and dish soap to help loosen build-up. If your grill has a grease tray, pull it out and wipe it clean, built-up grease can cause fires if not cleaned properly.
While your grill grates and deflector plates soak, use your wire brush to clean any loose debris in both the lid and bottom of your grill. You can use a small Shop-Vac to get the debris out of the bottom of the unit.
Pull your grates and deflector plates from the soap and water mixture and scrub them clean, then let them dry. Once dry, reinstall them and then move to the exterior of the grill.
Using a good all-purpose cleaner such as simple green, wipe down the exterior of the grill and clean all water spots, grease, and dirt, and grime from the front and back of the unit. Once everything has been cleaned well, you will want to do a quick burn to make sure everything is running well. Do this for 30 minutes before shutting the grill down.