Buy a humidity gage and know what is going on. They are cheap and any lumber yard, Target, Walmart, etc have them. You might be able to get one for $10, certainly no more than $20.
Then look to keep the humidity in the range of 55% or lower. Personally, I consider anything under 50% and above 35% to be 'perfect.' Above 60%, and you're asking for mold/mildew issues. All of the earlier mentioned ideas are good ones: ventilation, de-humidification, etc, especially venting in the bathroom during/after showers or in the kitchen while cooking.