i have a helmet cover that i really like in the winter although it looks dorky - some MTB friends told me it looked like i have a condom on my head, but as i said it works.

it's a thin windproof layer that goes over the helmet and is not waterproof but keeps the rain out well. i like it b/c it doesn't get sweaty since it doesn't touch my head and blocks the wind and rain (mostly). at first i had trouble using it with the visor on my helmet, but it works fine. although it does NOT work with my helmet mounted light, but i usually only use that for off-road, not commuting.

then i add the windproof headband.

when it's really cold i have a skull-cap to put under the helmet. with all 3 (helmet cover, headband, skullcap) i am fine to -20C/F