Empty the gas tank. Gas left in a mower over the winter can sit and as evaporation takes place the liquid turns into a gummy residue. This will cause clogging throughout the gas lines. Add a fuel stabilizer and run the engine to get it circulated. Allow the engine to cool and siphon the gas into a gas can for later use. Be sure not to mix regular gas and gas/oil mixtures. Start the mower and run until it stalls. Repeat this step to make certain all gas has been removed.
Disconnect the spark plug. Before taking any other steps, be sure that the spark plug is disco'd so as not to kick-start the engine. You will avoid serious blood loss.
Remove the blade. This will make it easier to drain the oil and provide an opportunity to sharpen the blade. It's much easier to sharpen when it's been removed.
Drain the oil. If the mower has a 4-cycle engine, you should change the oil. (Some mowers and most trimmers have 2-cycle engines, in which the oil is mixed with the gas.) Place a large piece of cardboard or tarp to catch any oil that splatters. Rest the mower on its side with the air filter and carburetor facing up, so oil and residual gas don't leak into them. Remove the oil reservoir plug and slowly tilt the mower until the oil begins to drain into the pan. This plug is in most cases located near the mower blade shaft. Replace the plug when all the oil has drained.
Clean the undercarriage. Use a putty knife and wire brush to scrape off the grass and excess mud caked on the mower deck. This prevents rust, clears the passageway to the discharge chute, and allows the aerodynamics of the deck to work as designed. This can be done regularly throughout the growing season to promote better circulation and draw on the grass blades. With the deck cleaned, reattach the sharpened blade. Once you've finished and can turn the mower upright, fill the oil tank with fresh SAE 30 or 30-weight oil, and recycle the used oil at a service station.
Change the air filter. A dirty air filter keeps the engine from burning gas efficiently by restricting the air needed for combustion. If your mower has a paper filter, replace it with a new one, paper edges facing out. If it's an oil-soaked sponge filter, remove it, wash it out with soap and water, allow it to dry completely, and then add a bit of clean oil to it before putting it back.
Remove and replace the spark plug, using a socket wrench with a spark-plug socket, which has a neoprene lining to protect the plug's porcelain casing. Even if the old spark plug is in good shape, for a couple of dollars a new one will perform better and ensure a smooth start come spring.