Some people just brush off the big chunks with a brass bristle brush. You can burn them off on a gas grill is you have one, then brush. If you get all the coals hot in your WSM before cooking, you can put the grates on top of the charcoal ring and burn them off, then brush.
I've heard of people using oven cleaner, but I wouldn't recommend it. I just soak in the sink in hot soapy water and scrub by hand with a grill scrubber available at the hardware store. It's like a ScotchBrite pad attached to a handle.
Regards,
Chris