I wash my horse's tail quite frequently and use any kind of clear leave in conditioner/detangler. Silverado, Cowboy Magic and Vetrolin all make one. My horse has a black tail so I don't use any white stuff in it - I find it makes it dull. I also wash his tail with a blue tinted shampoo.
I wash it, spin it around to get the excess moisture off, work in a bunch of conditioner/detangler and then let it air dry. I never brush his tail, I pick through it strand by strand to get the tangles out. In the summer I usually bag it to keep it out of the sun. You can see his lovely locks in my DP. That's almost a year ago so it's a little thicker now.