I'll second the importance of keeping feet and hands warm. When doing barn chores during cold conditions I always keep at least one extra pair in my pockets so I always have a dry pair to slip on.

While not waterproof these are great for working around horses or anything where you need a good grip. I machine wash mine in warm water and hang to dry.

http://gloves.lfsinc.com/content/?p=908

When it gets real cold I switch from Smartwood socks to Filson socks in either the medium or heavy weight. I've never found any socks that beat Filson when it's cold.