Don't live in a damp or cold climate is #1. We never have a problem in the summer.
down below either... We use a solar
heated garden sprayer in the cockpit's foot well.
Don't have wet towels or anything else down below, and keep the lid to the head
down. (Levacs are the best lid seal). This minimizes evaporation, which leads to condensation
Have a LOT of ventilation. We have "Vetus" electric
fans going, whenever we're cooking
. Numerous regular fans help too.
Wipe mildew right away. It multiplies otherwise...
If not under way, "descant cans" in nooks, crannies, and the bilge, help.
HAVE A BONE DRY BILGE! This requires a bit of effort, but is well worth it. It reduces hull blisters
By having a 4" mattress pad covered with "waterproof" cover, THEN a 2" memory foam "topper" with regular pad, the top cover & 2" foam can easily be put above decks to dry out on occasion. The waterproof barrier between the two pads keeps the bottom thicker mattress and structure underneath, from being perpetually damp and getting mildew.
In a marina, (yuck), you can resort to a dehumidifier! We resorted to one for NC winters (below freezing), and it really helped, but the dry skin became an issue.
By far the best tip is to just live in a climate that doesn't create condensation!
Hope this helps...