Ok so I'm on the other side of the world to you guys and no snow etc. But tails get pretty revolting here too!
I am a tail nazi, they have to be thick, shiny and tidy.
So my guys all get their tails bagged 24/7 with waterproof, zip up tail bags. They come off when I ride, get a light brush with a body brush, and then straight back in bags. I wash tails once every month, shampoo thoroughly, condition and leave the conditioner in for at least 10mins. Run a wide toothed comb through the tail in sections starting frrom the bottom and working my way up. Rinse out. Leave to dry and then spray with No-Knots or Cowboy Magic which repel light dirt and help to keep the tail tangle free.
My tails are always kept trimmed and squared at their base, cut off just about the fetlock joint. I don't like tails longer than this or not squared off, they look ratty!