Sgt. Rock has an excellent article about staying warm. Two particular jewels of advice he offers are 1) you can use an army surplus jacket liner as your "puffy layer" for about $10 - $15 and 2) you can keep your feet warm in running shoes with Gore-Tex socks. I would add that there are probably other sock options that will work equally well. Just be sure to pick a material that insulates when wet.