1) Always verify the weather before you go running
2) Long runs take water & food (Cold weather you still sweat)
3) Running in deep snow wear water proof clothing
4) Foot wear
Gortex Shoes - Keep your feet warmer and dry, but wants you sub-merge your shoe completely in water or snow your foot will become wet. (Shoe is design to keep water out, but not allowed the water to drain out)
I put bags over my socks and use rubberbands to keep the bags up if running in deep snow or water with the temperatures below frezzing.
Use slip on spike cleats, They clog in couple inches of snow, but great on ice.
Put Tek Screw in the button of your running shoes. It really works better than spike cleats and works on ice well.
I just used a pair of Yaktrax PRO. They work just as good or a little better than the tek screws for trail running only, but not for dry ashault roads (1/21/12)
Wear Smart wool or wool (Wool will keep your feet warmer than cotton socks)
Take extra socks if you expect your feet to get wet with the temperature under freezing.
6) Wear Layers - you can remove layers if you too hot.
7) Keep your face, head,hands, and legs covered to avoid frost bit.