keep your binoculars warm, I keep mine zipped inside my jacket when not in use, and have even gone so far as to sleep with them in my sleeping bag. also breath out your nose and not your mouth when looking through your binocs. its your hot breath hitting the cold glass that is making them fog up. good glasses have things done to them that help prevent fogging (nitrogen purging and lense coatings and such), but even the best glasses will fog up in very cold weather. I have also heard wiping the lenses down with vinegar helps, but I have never tried it, I didnt want the vinegar to have adverse effects on my lense coatings.