You need to find a person with a Morgan show horse and ask them, those horses have ridiculously long tails.
Otherwise from what I've heard and seen some horses have the "hair" gene and some don't. I've heard MTG works great, but I've never tried it. I almost never brush either manes or tails, just pic out the twigs & crud and brush them out when I wash them (w/plenty of conditioner). Trim it high enough so that you don't have to worry about them stepping on it and pulling it out when they back up. Soda has a great tail, but he seems to have the "hair" gene, so I don't think it counts. I trim 3-4" off several times a year and it's still fetlock height most the time and really thick.