Wall-mounted split
Rinse the filter every couple of weeks in heavy use, and keep leaves and clutter away from the outdoor body.
Ceiling cassette
Give the visible grille a regular wipe and schedule a coil clean before smells or weak airflow appear, since reaching it later is harder.
Centralized and ducted
Treat uneven room temperatures as an early warning, and have the ducting checked for damp before the wet months arrive.
Portable and window
Tip out the tray often and keep the filter clear so the small build is not pushed too hard in the heat.