As Paul pointed out, trim out the hard fat on the side. Besides that, I don't recommend trimming too much. Everyone has their own ideas on it, but since the wsm is a vertical smoker and heat rises, I only trim any really thick areas of the cap to make it a more consistant thickness.
I don't trim anywhere close to the end of the flat since it will be over the hot outer edge of the grate, and I save a couple of the thicker trimmings to lay on the grate under the end of the flat.