Grates are the exception to this general rule. They need to be cleaned completely after every use. This prevents food from sticking on your grill making it much easier to cook on. There are a lot of products on the market for cleaning your grate, but really any good stiff wire brush will do. Now, few people seem to know the best way to perform this little task. First of all, it is better done after you grill and not before. Once your grill has cooled down a bit, but is still warm, take your brush and clean off any remaining food particles. It is much easier to clean off food particles while warmth is still present and not after they have cooled and been allowed to harden on the grate. If you have a cast iron grate clean completely then, brush on some oil with a paper towel to keep your grate in proper shape. Bare cast iron needs to be inspected regularly because they are more likely to rust.
One of the great things about charcoal grills is that you clean it out after every use. However, many people don't take advantage of the situation and simply dump the ashes out. Keeping a good wire brush handy to sweep out the cooked on remains will make additional cleaning jobs much easier. And don't think that because you have one of those grills with the ash catchers that you don't have to do any cleaning. Everything you cook on requires cleaning. So every time you use your charcoal grill, clean the grate and brush out the ashes and cooked on foods. This will cut down on excess smoke and bad flavors. Periodically, get in there and really scrape out all that stuff that just seems to develop on the bottom of your grill. At least once a year, get out a bucket of soapy water and give it a really good clean. If you need to touch up the color get a can of grill safe paint. Just ask someone at the hardware store and they will know exactly what it is. If you do this, your grill will last for many years and produce great results.
The bad thing about many gas grills is that they have a setting on the knobs marked “clean”. Many people are mislead to think that this is how you clean your grill. Like a charcoal grill, you need to clean your gas grill every time you use it. Make sure that the grates are cleaned after you grill and brush off the sides and lid after every use. Use the clean setting to preheat your grill. Yes, it will burn up stuff that has fallen down into the grill, but it doesn't really clean it. Regularly you should lift out the cooking grate and clean off the barrier above the burners. This might be lava rock, briquettes, or some variation of metal plates. However, it should get cleaned of the cooked on grease and food particles periodically.
Once a year you need to do a good clean on your grill. This requires that you take portions of it apart. First, start by disconnecting the gas and then lift out the grill parts layer by layer. Once you get down to the burners, make sure you inspect them thoroughly. There shouldn't be anything blocking the flow of gas. If the burner is clogged it will give you uneven heat and make for poor grilling. If you can clean it, do so, if not, then replace it. If your grill uses lava rocks or ceramic briquettes you need to make sure that these are not too heavily encrusted with cooked on foods. If they happen to be, replace them to avoid bad tasting smoke that dirty rocks can produce.
With everything out of the grill, clean it completely with soapy water. If the paint is coming off, or if your grill is showing signs of rust then you need to do some painting. Other than that, put everything back in, check to make sure that all connections are good, and then turn it on. Let the grill heat completely before you cook again to make sure that any left over soap residue gets burned off.
There are shortcuts to cleaning a gas grill, but these are not always a good idea and there is one method that is definitely How NOT to Clean a Gas Grill.
Stainless Steel presents some special challenges to cleaning. Stainless steel will show every finger print, and low quality stainless steel will rust and discolor over time. I have some special instructions you can read on cleaning stainless steel.