Drain or Gasoline
Old gas is a big reason why mowers won’t start. Make sure to run all the gas out or drain the gas after use.
Check Oil
Check the oil regularly. Monitor the oil levels and look for debris in the oil or dark oil. If the oil looks dirty, replace it. When replacing the oil, make sure to drain the oil completely.
Clean Out Undercarriage
Grass can get caked on to the undercage blocking the discharge chute. Make sure to clean the grass off using a wire brush and any other access debris can be cleaned using a hose.

Inspect Air Filter
If the air filter is clogged or dirty it puts added stress on the mower and burns gas inefficiently. Make sure to check the air filter and replace it if needed. Air filters are relatively inexpensive so annual replacements to keep your mower running as smooth as possible are recommended.
Change Spark Plug
Similarly to the air filter, spark plugs are needed for a functioning mower. Since they are inexpensive, again, it is recommended to replace these yearly.

Sharpen Blades
Blades see lots of wear and tear and dull blades will pull and tear grass rather than leaving a clean cut. It is not recommended to sharpen your own blades. If your mower is in need of a sharpening bring it into a repair shop; it is often included in a tune up.
Get a Professional Tune Up
To get your mower ready for the season it may not be a bad idea to have a professional look at it. A repair shop will drain old oil, replace air filters and spark plugs and perform diagnostic tests.