I have used neoprene booties and they work okay if money is not an issue for you. But if money is an issue, try the plastic grocery bags over your feet (not very elegant, but it is hard to beat free in my book!). I have done that on several of the coldest days this year and I have been pleasantly surprised at the difference that they have made when teamed up with a pair of wool socks. I initially tried them one day when I froze my toes on the way to work and needed to find something at my office just to get home. Now I'll always keep a couple of bags in a drawer in my desk, just in case!