As long as I am moving, I haven't had an issue at -40 wearing Filson mackinaw wool. My Kifaru Lost Park Parka over some everyday sweaters has been great too. Normal (eek: cotton) Wally World sweats for a leg base layer has always been fine.
Boots? Muck boots or some sort of packs. My Kennetreks have been fine though I am not a normal Kennetrek boot fan. The bunny boots LoadClear mentioned work well but I haven't found them to be comfortable to walk around in. You don't have to worry about rain or moisture when it gets really, really cold, other than from yourself sweating.
Cabelas Stand Hunter Gloves have been fine though if you need any kind of dexterity, they need to come off. I am still looking for a pair of gloves that will allow me to work with my fingers in that kind of cold.
Hats? Something that covers your ears as well as your cheeks (mad bomber style) are nice. It doesn't need to be some crazy space age material. If you have any kind of a beard the ice will build up due to your breath and it'll hurt like hell to open your mouth, as it'll yank your whiskers out.
I have had more issues with gun parts freezing solid than I have with inadequate clothing. Once it "warms" up and moisture comes into the equation, things change.
