I'd tried everything years ago (not cycling back then, walking still had my feet freezing in <30 F). Then I got a hand-me-down of a small pair of men's tall rubber boots. Being too big for me, I wore them with a thick pair of wool socks with thin socks underneath, and voila' -- no more cold feet. At one point I was out taking pictures and went and stood in the water of the Chesapeake Bay after a snowstorm in January and didn't feel the cold water at all. A few years later I started wearing Crocs at work, and noticed that my feet stayed warm with no socks at all until it was absolutely freezing out; even then it was because of exposed skin at the back. Thick rubber or rubber-type material blocks the wind and really insulates my feet from cold air better than anything else I've tried.