Try to take an off-season cruise or two. If you don’t do this, go to the boat and work all of the systems. Is the furnace working, or does it need service? Is the AC water heater working and not leaking? Use the shower to test the water heater. If you are in a southern climate, see that the air conditioning is working. Even if you have a diesel furnace, run it to heat the boat. Stay overnight a couple of times, even if it is at your marina.
Keep your boat clean. Start with the canvas. If it is growing green on the outside, or if other stuff is on the inside, get it cleaned. Pull the Bimini and get it cleaned and coated with 303 or another waterproofing product. One of the best cleaners is Iosso. If you soak your canvas in it, the mildew and droppings will be attacked by a biodegradable solution.
Wash your boat at least once a month; I use Mermaid’s boat wash with Carnauba. We wax twice a year — one time includes the hull and the other time is just above the hull. A good time to wax the hull is during the haulout. We also wash and “wax” the windows, using Mermaid’s Cleaner and Polish, inside and out. We put on a little bit extra, let it dry and then clean off the extra. For all isinglass or other clear surfaces, we use soft cotton cloths. Never use paper towels — they will scratch the surface.
The rest of our focus is cleaning the inside and keeping it dry — carpets, all surfaces, wood, vinyl, sheets, etc. It makes our boat smell great (if the head system is in good shape). Putting in a dehumidifier is a good idea.