Start with warm, dry boots, warm feet, and don't impair your circulation by tight boots or socks that are too small. Use a thin sock liner with a medium weight wool outer-sock. This provides wicking, insulation, and cushion. Choose boots that fit well but are not the least bit tight. Even slight pressure will cut off circulation and result in cold feet.
Good winter riding boots have an inner layer of perspiration-wicking material.
Often the insole is removable to allow drying. The middle layer should act as a mediator between perspiration and the snow and slush. The outer layer should be durable yet breathable. If your toes get especially cold, you can use tapaderos which are stirrup covers. If your riding doesn't require intricate rein aids, use fluffy, lightweight polyester-filled gloves.