I always thought I could handle the cold, then I moved to Kansas and learned to sit in a tree in December and January. I quickly realized I'm not that tough.
I've learned that if I don't start out the morning right, I'll be miserable all day. I need to eat in the morning, not some skimpy bullshit meal either, a full on breakfast. That helps get the body going to get warm, like fuel for my truck. I try to prepare this the night before and throw it in a tortilla so it can be microwaved the following morning.
Secondly, I run the heater much warmer than I need to and have my extra layers out off and exposed. It also continues to warm me. Once I get to where I'm hunting I'll put my warm layers on an have a next to skin dry shirt in my pack. It's a pain in the ass to change back and forth, but it sucks worse to freeze.
I'll sit on a foam pad when I glass. I also take my boots off and put my down booties on. Boots go in my pack.
I also eat quite a bit and drink lots of warm beverages.
This is my system when I'm not lazy and decide I don't feel like just walking all day.