If you are using glass tanks I've heard that a piece of plexiglass for the top works great. Just drill a few holes in it. I live here in Michigan and winters were always such a pain in the butt when it came to humidity. Now that I've switched over to racks, humidity isn't an issue.

Also remember that heat lamps are humidity's worst nitemare. Some sort of under the tank heater works much better. Hope that helps