Brush the grills, dump the ash & clean out any greasy gunkiness from the charcoal bowl, replace the top foil layer over the water pan. I will sometimes leave the grates on my Performer and burn 'em off when grilling, then spray with Pam and put back in the WSM. I store mine covered with top vent & one bottom vent open. Never seen any mold.
Traeger Lil' Tex Elite, WSM, Green SS Performer, 22" OTG, hyperfast orange Thermapen. CBJ.