Aim for the humidity, not just the mold
On the mainland, mold prevention is mostly about stopping leaks. On Oʻahu you also have to fight the ambient humidity, because the air itself will grow mold in a still, closed space. The target most pros suggest is keeping indoor relative humidity under about 60%, and ideally closer to 50%. A cheap hygrometer (humidity gauge) is the single most useful tool you can own here — it turns “it feels muggy” into a number you can act on.
The high-impact habits
A few things do most of the work. Run a dehumidifier in the dampest rooms — closets, bedrooms, an un-air-conditioned back room — sized to the space; it's the most reliable way to hold humidity down in this climate. Use your AC to dehumidify, not just cool: running it a bit longer at a moderate temperature pulls more moisture than short cold blasts, and a properly sized unit matters. Ventilate the wet rooms: run bathroom and kitchen exhaust fans during and after use, and make sure they vent outside, not into the attic. Keep air moving: open jalousies for cross-flow on breezy days, run ceiling fans, and don't let closets and rooms stay sealed and still.
Watch the usual suspects
Give a little regular attention to the spots that fail first. Squeegee shower glass and tile; don't let damp towels and laundry pile in closed hampers; leave a little air space behind furniture on exterior walls; and check under sinks for slow leaks. Service your AC — a clogged condensate line is a classic hidden mold source — and clear gutters and downspouts before the winter rains so water sheds away from the house. For post-and-pier homes, make sure the under-floor can breathe and isn't skirted airtight over bare, damp dirt.
Before you leave town
Homes that sit closed and un-air-conditioned while owners are away on the mainland are a top source of the “came home to a musty house” story. If you'll be gone, leave a dehumidifier running, set the AC to a moderate “dry” or humidity-holding mode if it has one, or have someone air the place out. A little airflow while you're away prevents a big remediation bill when you return.
When prevention isn't enough
If you're doing the basics and still fighting recurring mold, there's usually an underlying issue — a hidden leak, an under-floor moisture problem, poor ventilation, or a home that simply can't dry in a very wet microclimate. That's the point to get an inspection that reads the moisture and finds the source.
Choosing and running a dehumidifier
Since a dehumidifier does most of the heavy lifting in this climate, it's worth getting right. Size it to the space and the dampness rather than buying the smallest unit available — an undersized machine runs constantly and never reaches target. Look at the pint-per-day capacity relative to your room size, and consider a model with a built-in humidistat so it cycles to hold a set level rather than running blind. A continuous-drain hose is worth having if you can position the unit near a drain, since emptying a full bucket daily is the reason most dehumidifiers end up unplugged in a closet. Keep the filter clean and give the unit clearance for airflow.
A seasonal checklist
Ahead of the wetter months, roughly November through March, a short round of maintenance pays for itself: clear gutters and downspouts and confirm they discharge away from the house; check the roof and flashing for anything loose before heavy rain tests it; service the AC and clear the condensate line; confirm bathroom and dryer vents still terminate outside; look under the house or in the attic for anything that's changed; and check that ground still slopes away from the foundation. In the drier months, focus on airflow — opening the house on breezy days and airing out closets and closed rooms.
Spot-cleaning small growth safely
For a small patch of surface mildew on hard, non-porous material — tile, glass, sealed surfaces — careful DIY cleaning is reasonable. Ventilate the area, wear gloves and a mask, use a detergent solution rather than dry-scrubbing, and dry the surface thoroughly afterward. Skip bleach on porous materials, where it leaves the roots behind and adds water. Stop and call a professional if the area is larger than roughly a few square feet, if the material is porous and soft, if it keeps returning, or if anyone in the home is medically vulnerable.