What are the most important things I should do to maintain my air conditioner before summer?

Change the filter and hose down the outdoor unit — that covers 90% of what matters. The filter is cheap insurance for airflow and efficiency, and the outdoor coil gets caked with dust and pollen. Use a garden hose, not a pressure washer. Make sure the unit is level and the drain line isn’t clogged. Skip expensive tune-ups unless the unit is old or acting weird.