Boots rated for -100 degrees are quite comfortable when the temperature is in the teens or below, especially if you're standing on cold ground or snow, but not if they've become moist from being worn in the car on the way to the site. Always remember to take the liners out of pac boots after an observing session to allow them to dry out, especially if you plan to observe the following night. Keep well hydrated but avoid caffeine. Caffeine is a diuretic and reduces blood volume, making it more difficult for the body to maintain a normal temperature. I usually take bottled water and a half-gallon of decaffeinated coffee with me on cold nights.
Cold conditions are often very dry conditions.
Don't forget the lip balm, etc. I often find that any exposed skin gets dried out during the winter. A little moisturizing lotion when I get back to the house usually helps to keep my hands from becoming too badly chapped. Chemical handwarmers are another good item to have, but aren't a substitute for wearing adequate layers of clothing. My parka has two pockets inside over the kidney area that are designed to hold handwarmers.