If they are showing some wear, don't try to clean them. It's cheap insurance to simply replace them with new plugs. Be sure to blow away any debris with compressed air before removing the plugs, and torque the new ones to specifications in your owner's manual. A few dabs of anti-seize compound will ensure easy removal next time. Clean and lubricate the choke and throttle linkages and apply grease to all the fittings, including the mower deck.
Replacement is needed when there are any deep nicks or cuts in the blade's tempered surface.
Inspect drive belts and tighten loose belts. If your inspection shows any deep cracks or ply separations, replace the belt. Apply grease to the posts to retard corrosion. Last but not least, change the oil in the crankcase of the engine. Change the filter.