Of everything a homeowner can do between professional maintenance visits, keeping the filter clean has the biggest impact for the least effort.
Most wall-mounted split units have a filter that slides out from the front panel without tools. Every four to six weeks — more often if you have pets or run the unit constantly — slide it out and check it against the light. If dust noticeably blocks the mesh, it's time to clean it.
A soft brush or a vacuum's brush attachment removes most surface dust. For a deeper clean, rinsing gently under lukewarm water and letting it air-dry completely before reinserting works well — a still-damp filter can encourage mold growth inside the unit.
This is one of the few maintenance tasks that's genuinely safe for a homeowner to handle solo. Anything beyond the filter — the coil, drainage line, or electrical components — is best left to a scheduled maintenance visit.