For really cold weather, I use a neoprene half-mask face balaclava (one that comes up and mostly covers my ears), and an exercise ear-band from target (C9 brand). Usually, even down close to freezing, I still want the top of my helmet uncovered so my head heat can vent though the holes...It's my ears that get cold, and the balaclava plus ear-band gives me 2 layers on the ears.

Down into the 20's, I will add some kind of skull cap to cover the top of my head.