All my years in cold climates don't matter much down here, but here's some things to keep you feet from freezing while riding in cold weather. Feet are actually easier to keep warm than the fingers.
1. Keep the body (torso) warm or the extremities suffer (don't neglect you head either)
2. Don't wear tight shoes or boots. Looser so that your thick, heavy wool socks (or whatever you're wearing) aren't tight on your feet and have room to hold warm air in the fibers. Not so loose that your footware is slipping. Just "snug".
3. Now the big item. As much as you might love to ride bareback it's not helping keep your feet warm, because you don't have stirrups. Putting regular pressure on the bottom of your feet in the stirrups (posting is great for this) helps pump more blood (warm...from the torso) into your feet. Same concept as being on foot. If you stand around your feet freeze, but if you walk around and keep the feet moving you can stay warm.
4. When you layer make sure you can strip off items and still remain warm enough. If you start to feel very warm (even on your feet) take off something from the torso. As the body cools a bit feet and hands will too.
Oh the memories of riding in snow and freezing weather. While I love the roughly 8 months of lovely riding weather we have down here (what's snow?
) where there's no such thing as single digit weather (even upper 20's at night is is not common) I'd gladly take a month of really cold weather if I could get rid of July with it's triple digit weather.