Wall-mounted split
Rinse the filter every couple of weeks under heavy use, and keep leaves and clutter clear of the outdoor body.
Ceiling cassette
Wipe the grille often and book a coil clean before any smell or weak airflow shows, since access only gets harder later.
Centralized and ducted
Watch for uneven room temperatures as an early sign, and have the ducting checked for damp before the wet season.
Multi split
Keep each indoor filter clean and the shared outdoor body clear, so one neglected room does not drag the rest.