chemical toe warmers, or if you can fit them, hand warmers taped in place. I use a sock liner, put the warmer on top of toes, then put on the over sock.
they are good for about an hour before then need air to reactivate, when starved for air they stop working.
which is a good thing cuz if you have a 1 hr commute, by time to get to work they are shutting down - you can then put them in an air tight baggie for the day and use them again on your way home!
if you go out for a longer joy ride, then you find yourself stopping at a dunkin donuts to warm up your feet and reactivated the chemical warmers.