Like the rest of them, I go all winter long on a regular basis, the best sailing is just coming up. Aside from that, I keep all or at least most portlights open (I have plexiglass shields on each one that keeps the rain out but lets the air in). When on shore power, I keep one of those tall electric oil filled heaters going on low and a de-humidifier. I put on my heavy winter docklines and large fenders. That's about it and I've never really run into a problem. Mold is the biggest issue, you need to keep it out by having lots of air flow and a bit of heat.
Oh ya, I spray parts of my boat, particularly the hanging locker and other potential mold breeding grounds with this natural product:
At Home - How It Works