I don't bike in the cold (too much snow) but I run until it gets below - 30. The best way I've found to keep your feet warm is to keep your feet cool. If you put hot pads in your shoes and they get warm enough to make your feet sweat, you'll be cold. The moisture sucks the heat out of your skin. I have a pair of shoes that are a size too big. When it gets cold, I wear a wicking sock (made of material that wicks the sweat away from your skin) next to my skin and then a double layer running sock over top. Put on a pair of wind proof shoes and I'm good to go.