I use tart from the bottom and work my way up. I divide the hair into short (about three inches) sections and go from there, little bit at a time. It takes me about half an hour normally to brush my horses tails, and I only brush their manes and tails about once every two or three months.
Usually I just pick through their hair with my fingers, get the worst of the weeds, sticks, and burs out, and untangle the bad tangles.
Both of my horses tails are long and thick. My mare's is a little shorter (about even with her fetlock joint) but is thick enough for two or three horses, and my geldings tail is thick enough for two horses and lacks about an inch from touching the ground.