If you expect to be caught in a frog stranglin' downburst, carry a gallon size freezer bag and use it to cover your feathers. After all, if the rain is really coming down, you're not going to risk a shot anyway because the rain will wash out your bloodtrail.

Use a waterproofing agent on your feathers. I just spray my feathers down with Scotchguard the night before hunting if it's a wet forecast, but there are specialty feather waterproofers on the market.

Even in a light drizzle, it's best to keep your feathers covered and only load an arrow when you are pretty sure the deer is coming in and is likely to offer a shot. I know it's all the rage now to load an arrow on the bow and then hang the bow in the tree for immediate access. It's really not necessary to do that.