Good nutrition is important for growing a healthy tail. There really is no other way to make the tail grow, but there are some things that you can do to help prevent damage and hair loss.
To help prevent tail rubbing a regular deworming schedule, regular cleaning under the tail and conditioning at the top of the tail so that the skin doesn't get too dry are important. I like to use baby wipes to clean all the areas under the tail each time I groom.
I brush out the tail every time I groom as well. I know a lot of people don't like to brush the tail out regularly, but I have had no problems doing so. Each of my horse's came to me with tails that were thinner that they are now, and now their tails are pretty nice. I have over the years tried many different products on my horse's tails and the only one that I like and will now use is Miracle Coat Horse Leave In Conditioner Spray. It is all natural and has tea tree oil in it. It leaves the hair the most natural looking and soft and helps to keep it easy to brush out, also doesn't pick up dust like some do. I don't use it everyday, but will spray it in if the hair is starting to feel and/or look dry or if my brush isn't going smoothly through the hair when I start to brush it out. For brushing, I like to use the Oster style mane and tail brushes and I start at the bottom and work my way up little at a time and hold the hair above where I am brushing so I don't pull any out, then I may go through with a wide tooth plastic comb. A wide tooth plastic comb is best after a shampoo and conditioning when the hairs are wet, used in the same way as the brush, it won't pull or break the hairs. When used this way carefully you shouldn't be losing tail hairs.
I have never braided/bagged my horse's tails, so can't comment on that, but their paddocks get really muddy in the spring and I just use the same routine as above, but if the hairs are really super muddy, then I will hose the bottom of the tail off and then spray in the conditioner and comb out gently from bottom to top. Remember to hold the tail hair above the section that you are brushing or combing it will help to keep you from pulling hairs out.