If you have very close fitting road shoes, then I find merino ski socks are good. The extra height up your calf keeps things toasty.
If you only have summer shoes with vents, then shoe covers keep the wind chill at bay.
The secret weapon is to put some charcoal toe warmers in. If you have enough room in the shoes, put some silk liner socks on, then warmers on top of the toes, then ski socks. If that’s too tight, just stick the warmers on your toes and pop the ski socks on.
http://www.amazon.co.uk/Hot-Hands-Foot-Warmers-pairs/dp/B000YEZLD8/
Don’t do you shoes up too tight, as it will restrict your circulation.