What they said (wool socks plus toe covers or booties), plus make sure your shoes aren't too tight. Especially if you bought your shoes in warm weather with thin socks on. Less insulation with more CIRCULATION is actually warmer than the other way around... of course if you have both circulation AND insulation, your toes will be warmest.

In a pinch down to 35 or 40° F, you can just put plastic bags over your socks, inside your shoes. They don't take up any space. Just make sure you're wearing wool socks (to wick sweat away from your feet) and you're not riding with anyone who's going to make fun of the plastic hanging out of your shoes.