Short of indoor storage, the most successful off-season cover is one that protects the boat from precipitation, dirt and damage while allowing sufficient ventilation to prevent mold, mildew and musty odors. Plastic shrink-wrap seems to be today's solution of choice, but it must be properly installed and outfitted.
Boats that are painted (Awlgrip, etc.) should be shrink-wrapped carefully to ensure that the wrap, its seams and tie-downs don't come into contact with painted surfaces, which could be damaged by abrasion as the cover moves in response to wind and temperature changes.
Since shrink-wrap provides such a good seal, make sure the installer provides screened vents facing in all directions to facilitate airflow. You'll want to have a door installed, as well. This way you can board after the boat is wrapped and open as many ports and hatches as possible for ventilation.