A couple times a year, I take some soap, water and a nylon brush and scrub the inside of my WSM. I don't really try to clean it. I just try to get the loose stuff off and then give it a rinse. As for the flaky stuff on the inside of the lid, I use the same brush and just brush it (dry). The stuff on the sides I'm not at worried about. The stuff on the lid can flake off and get on your food. That's worth worrying about.
About the only other thing I worry about is the lip on the middle section that the lid sits in. If you don't clean that gunk off, the lid can literally stick to the middle section. I just take a paper towel with white vinegar when the unit is warm to the lid and the lip. Works pretty good and is food safe.