The best way to prevent your horse from getting frostbite is to keep it in good health, be sure it has lots of good hay to eat, water which is essential to keeping your horse's internal furnace running, and a draft free place to get away from the cold and wet. But again, frost bite is very rare and unlikely to affect a healthy horse. I live in an area where the winter temperatures can dip below -4F (-20C) and have never owned a horse, nor have seen any of my family's horses get frostbite.
If you suspect your horse has frostbite, call your veterinarian who can administer an anti-inflammatory, pain medication and antibiotics to prevent infection. While you're waiting for the vet, warm your horse's ears the same way you would a human's.
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