Larger size, so you can fit the proper socks. Spray anti-perspirant on your feet, to keep them from sweating. Thin silk or poly-pro sock liners next to your feet Wool, Smart Wool, or some PolyPro socks. Thick. Two pairs are better than one. I am in MT. We laugh at 30 degree weather. I have taken my Cabelas waders off and stood them up against the truck in -20 weather. Here is the thing. You feet do not circulate like other parts of your body. They do not move much, to generate heat. Your ENTIRE body needs to get the blood pumping to warm your feet. At some point, you have to MOVE, to get warm. You can try artificial heat. Some like the toe warmers. I have also used the full foot style. Can't say they make much difference in really low temps. I have also tried the battery operated insoles. Used em a few times, didn't really work for me. I sold em here, and the buyer is happy. Wearing the thick socks might be the reason. I have the same style (small lithium battery) in gloves, and they work great. Really thick insulated boots are clunky to walk in, heavy, and give you minimal extra time in the field. Thick socks breath, and help keep your feet dry.