Must keep head warm! I wear a scentlok base layer head mask which covers my nose and mouth. This keeps scent down but also keeps you warm because your breath warms up your entire face and head. On top of this, I wear a wool camo ski mask. It has extends to cover my entire neck. Cold air down your neck or back will kill you.

Also, feet must stay warm. My toes used to freeze because I would cram three or four pairs of cotton socks into a boot that was too small. You gotta have big roomy boots and nothing but thick wool socks. Also, layer with the wool/polyester blend long johns. I like at least three layers - no cotton! And I hunt standing up most of the time because your muscles keep you warmer. I like to hang my bow and keep my hands in my pocket until I see deer. This really helps too.