My brother recently taped a clear plastic bag over the top of his helmet to keep the wind out for these cold months. I figured that I would buy him a helmet cover for christmas, and I visited two LBS only to find that none of them had them in stock. Apparently they're not very popular anymore. I've never used one so I'm not sure if they're made specifically to keep just the rain out, or if they keep your head warm as well.
So now I have some choices. I want to get something head related that's good for freezing conditions. Within a range of 40-0 degrees. He has a universal fit Giro helmet with a visor, and there's enough room under it for a wool hat, so a skull cap would fit fine. He also wears glasses, so I think the balaclava is out since I've heard they don't work too well with glasses.