I also agree with the thick socks, some of those rental boots may not be as warm as you'd like, also a warm pair of gloves is a must
wool retains something like 80% of its insulating properties even when wet. this is good info as i find my feet sweating a lot.
i find under armor and long john type clothings are the best for underneath. im surprised it hasnt been said, but dress in thin layers. you reatain more mobility this way than a single thick coat and you can always shed one or two layers if you feel the need. once you get moving your fairly warm, falling and picking yourself back up tends to be a good workout. :P