Depending on your lens, you might be able to purchase a neoprene Lenscoat. I had to make one for my 300mm f/4 AF, as it the version before the 2001 AF-S. I also made one for the 105mm VR, as I had plenty of fabric left over. I did this mainly to help with the cold radiating into my hands in extreme weather. If you purchase some Nikwax spray, you can then waterproof the neoprene (don't spray it when fitted to the lens). This will help repel sleet and rain, as well as any snow that find it's way onto the coat and melts when indoors.
I went to a Canadian provincial park in -37c and could barely stand to hold the camera with my gloves on, because the magnesium alloy body was like a block of ice. For this reason I bought a silicone body cover for the D800, which just adds another barrier between the cold and your hand, whilst still being able to use all the controls on the camera. Mine is camouflage patterned, as is the neoprene for the 300mm f/4. It just helps break up the camera profile and blend into the surroundings a bit better, which if you are shooting wildlife can make a big difference. The frigid air temperatures also caused the grease in my lens to congeal. Nothing I could really do about that though lol.
You can also purchase nylon rain covers. These simply slip over the lens and camera body to keep the water off. You can buy larger ones that cover your head and hands too. They are readily available on
eBay and Amazon for a few dollars for the basic ones. That's all I have and it does the job. Ensure your bag has a rain cover. If not, you can purchase those separately too in various colours including camo. Keep a small tea towel (type you dry dishes up with) in your bag. If it belts it down with rain, you can at least dry your camera off when returning to the vehicle or shelter. A lens cloth will get soaked in no time.
Save the silicone packets that you get in vitamin and drug bottles. I have several of these that I leave in my camera bag. They help absorb moisture and help protect the lenses when stored away. You can also purchase these from
eBay and Amazon too.
As mentioned above, keep spare batteries next to your body. I normally wear a few layers, long sleeve thermal t-shirt, sweater, micro fleece, coat. I keep the batteries in ziplock bags in the micro fleece inside pockets. This keeps them warm and dry. You don't want to sweat onto the batteries.
Good quality gloves are important. I wear thin Northface ones, under mitts in extreme cold. That way you can still shoot and then replace the mitten when moving again. You can get mitts that flip the end up, so that more of your hand stays warm. You can also purchase little heat pouches that contain chemicals. When you crush the pouch the chemicals mix and create an exothermic reaction, which warms your hand. Great for slipping inside a glove or mitten when it's really cold out.