I'll take a stab at it:
1) Change oil and filter
2) Either treat gas with stabilizer or drain
3) Start mower after treating the gas (to move the treated gas into carb) or draining the gas (if drained, be sure to use the choke to totally flush the carb)
4) Grease all bearings and zerks
5) check and clean air filter (replace if necessary)
6) Put mothballs near air intake, engine fins, etc (keeps critters out)
7) Replace spark plug if needed. Put misting oil in the spark plug hole and manually move the cylinder to spread the oil around.
8) Remove battery and store in a warm environment. Use a trickle charger if you desire.
9) Sharpen all blades and be sure to balance them
10) "Wash" and wax your machine to prevent rust and other deterioration. Use water if you feel that it won't harm your machine (be sure to dry it thoroughly) Also, put grease or petroleum jelly on any expose metal parts to prevent rusting.
11) Clean around the engines cooling fins to clear any grass clippings, etc.
12) Clean and paint your mower deck!! Adjust and level upon reinstall.
13) Check all drive belts, mower belts, etc. Replace as needed.
14) Remove any accumulated debris (sticks, stones, leaves, etc) from under mower.
15) Cover with a soft tarp or lawnmower cover.