Do a search and you will find a long list of things you can do.  For me, the most important thing to clean other than the grates, water pan (dispose of grease) and dumping out the ash is to clean the lid.  Two areas on the lid are a problem if not cleaned.  First, after awhile, you may find that there is a flaking issue inside the lid.  If those flakes are not brushed off (a nylon brush is enough) they will drop onto the food - not very appetizing.  The second is the rim of the lid and where it sits in the middle section.  When the unit is warm, I take a paper towel with some white vinegar on it and wipe the lid and the ring on the middle section.  If you don't do this, the lid will stick to the middle section and it will not come off.  If this ever happens to you, the best advice I've got is to fire up the unit.  The heat will soften the goo and the lid should come loose.  The one time it happened to me, I pried it off with a screwdriver.  Thankfully I didn't damage the WSM.  (note: that was before I heard of or thought of firing up the unit).