Spring: before the heat
Replace the filter, clear debris from the outdoor unit, test cooling on a mild day, and pour a cup of vinegar-water down the condensate line to prevent clogs.
Fall: before the cold
Test heat early, replace the filter again, check that vents aren't blocked by furniture, and test your carbon monoxide detectors.
All year
Keep return vents clear, note any new noises, and watch your energy bill. A sudden jump with no change in weather is an early warning sign.
General information only, not legal or professional advice.