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The best way to prevent frostbite is to make sure that your coop is well ventilated. Ventilation is extremely important! Moisture collecting in the air leads to frostbite and other problems. Make sure there isn't any draft, but you need to have plenty of ventilation.
In my experience, vaseline on the combs doesn't really work. All it has done for me is drip down the birds feathers, creating a sticky mess. But, others have had success. You may want to try that.
One other thing that can cause frostbite is water getting on the comb or wattles. I know that one reason why my birds got frostbite in the past is that they would dip their heads down to drink and the front portion of the comb would get wet. This, coupled with cold temperatures, lead to frostbite. I switched to nipple waterers, which don't cause this problem.
Sometimes, there's not much you can do to prevent frostbite. Large combed birds may get it even if precautionary measures are taken. It is best to keep cold hardy breeds with small combs in cold climates. One of my breeds, the Wyandotte, is cold hardy and I haven't had any problems. But my bantams with large combs did have problems.
My bantams haven't had problems with frostbite since three years ago. That's because I began heating the coop. Many people don't heat the coop, and it isn't always a good idea. But, for me, that was the only way to stop frostbite. I had large combed breeds and the temperature got down to below zero.
With that said, be careful when heating! I tried heat lamps, but I think they almost increased the problem, and didn't help. The heat lamps only warmed the birds while they were beneath the heat lamp. They didn't stay under them the whole time, so still got some frostbite.
I hope this helps!