There's already so much to think about when you're choosing your gear for a winter run. Will this jacket keep me warm enough? Too warm? What about shoes? Traction aids? Once you've dialed in these basic choices, there are three more items that can make a tremendous difference in how much you enjoy your run: sunglasses, skin protection, and gaiters.
Sunglasses: Personally, I find that choosing the right eye protection is more meaningful in winter than other seasons. The snowpack reflects light, making even overcast days brighter than in summer. Snow blindness is a serious condition resulting from sunburned corneas, a consequence of all that UV light making its way unfiltered into your eyes, so you'll want to make sure that you're getting UV protection for your eyes.
I also find that the right glasses offer bonus protections on winter runs. First, the glasses create a small insulated pocket of air right in from of your eyes. This prevents the dryness, tears and icing that can make a cold winter run uncomfortable. I'll use clear lenses for this protection on dark days or night runs. Second, and maybe more importantly, the physical protection of the lenses will blunt any blows to your face and eyes. It's easier to spot thin branches obstructing the trail in summer, with leaves on, but without leaves, some of them are nearly invisible in the winter. I've taken enough whips to the face to be grateful for my glasses. I never trail run without eye protection, but that rule is even firmer in the winter.
Skin Protection: The same winter overabundance of UV light that can burn your eyes will easily burn your skin too. I'll admit to being lazy in the past about sunscreen in winter, and as a result I've had some gnarly sunburns. As the days get warmer and you have the chance to shed layers as you warm, make sure that you apply sunscreen to any new skin you'll expose along your run.
Skin protection also means creating a barrier to prevent chapping or, worse, frostbite. Many of the products that you are already using as an anti-chafe strategy should create a moisture barrier to protect your skin from cold exposure. Squirrel's Nut Butter is great for this, and Dermatone offers UV protection in addition to a moisture barrier. In a pinch, you can even apply your chapstick to your face. You might feel silly in the moment, but trust me, it's better than the regret that can come later.
Gaiters: What good is finding the perfect wool socks and Gore Tex trail runners if you still get soaked? When we trail run over snow, especially on powder, we create a little cloud of snow around our ankles. Any snow we contact from the air or sound will freeze to exposed socks around our ankles and create those ice pills that then melt and soak your foot inside your shoe. Spandex doesn't build ice pills the same way, so a minimalist gaiter like Dirty Girl will offer a lot of comfort with minimal cost or effort. Focus on protecting your ankles and don't bother with more expensive knee-high gaiters--if the snow is that deep, you're post-holing and trail running is the wrong sport until the snowpack firms up.
There's a lot of great advice about how to choose layers to stay warm (but not too warm) on a winter run, but less attention generally paid to these three items. Once you add them to your winter running routine you'll be far more comfortable on the trails in the elements. Discomfort from the cold can be easily managed--don't let it prevent you from getting out there!