Wow, I can't believe how many people stand behind their horses. I guess I just learned to never do that and it's ingrained now. But I can't figure out how you would even brush the tail from behind. There is nothing to brush against that way. I always stand to the side, always touching the horse. I do it much like Vidaloco does -- start at the top, then take up the bottom in my hand and brush out the snarls from the bottom up, until I meet where I left off from the top.
As for not brushing the tail so it stays full -- an easy fix for brittle hair, aside from hair products, is to use a stiff body brush instead of a tail brush or comb. It will get the worst of the dirt out, and if you brush it regularly, the tangles won't get as bad as if you just leave the tail be.
All that said, on the weekend I braided my mare's tail because it's too long and I'm really tired of brushing out the mud. Last spring I cut a few inches off, but it really does take an awfully long time for it to grow back. I figure I'll leave the braid in until it dries up around here -- just in time for fly season so she'll have her tail back!