Remove the mower drive belt and store to prevent hard spots from developing in the belts where it wraps around the pulleys.
Lubricate all pivot points and grease blade spindle bearing to prevent rusting from condensation build up over winter.
Drain fuel tank and carburetor or use a fuel stabilizer in the carburetor.
Clean the bottom of the deck to remove all grass build up. Use a wire brush and a putty knife and remove as much build up as possible then apply a spray on dry film lubricant to prevent corrosion. This will also help prevent future grass build up next season.