We don't do anything special at all with tails here. Our horses are out 24/7 in all sorts of weather, and in snow, rain, mud, whatever. We don't even comb them out all that often--not at all through the winter, more the rest of the year. Most get really long & we end up trimming them to keep the horses from stepping on them--there's nothing more useless than a dragging tail, really. Most of the horses have nice full tails too. We do have plenty of white/silver tails and yes, they do get quite dirty, but they wash out just fine--we showed two palominos this summer and it took very little washing to get their tails snowy white. I have a couple horses with naturally sparse tails but a bit of volumizing gel when we want to show them works wonders.
And, my all around favorite product for combing out manes and tails...even filthy tails that haven't been combed out all winter...is Show Sheen moisturizer gel. That stuff is amazing. Just a little bit rubbed into the most tangled mane or a tangled, dirty tail and it combs right out, slick as can be. You'd think the the mane & tail had been combed out just days ago.