First off, horses like humans have good hair genes or not so good hair genes, but all horses can grow a better mane and tail that is healthier and prettier. I want to share with you some of my secrets.
1.) Never brush or comb a tail that is not clean or is extremely tangled. You will end up brushing those tangles out but also pulling, breaking and ripping hair out at the same time.
2.) Wash about every 2 weeks and not more than one week. (You don't want to wash out or strip the natural oils in the hair). My favorite shampoo is actually one used on horses but also on livestock, it is called Orvus. Don't be surprise it is a white solid when it is cold and then turns to a milky sticky liquid when warm and then a clear runny liquid when warm but seems to be the best soap I have found for really sud-sing up and getting your horse clean! It can usually be bought in a one gallon containers and because it works so well, a little goes a long way.
3.) After washing your horses mane, tail and forelock then put your conditioner in. I actually use a combination of Mane N Tail conditioner (by straight arrow) and Cowboy Magic detangler together and it really seems to help get all of the tangles out. When brushing out those tangles, start at the bottom and be VERY patient and carefully work through the hair to ensure it is completely tangle free.
4.) Avoid using products like Show sheen which works great to get the tangles out but has a lot of alcohol in it so if you use it to get tangles out before be sure to wash it out and apply the products I outlined above.
(I learned this after MANY years of using show sheen on my show horses and just leaving the product in, after some time I began to notice my horses hair got very brittle and was easily broken and frayed. So though I still use Show sheen, I am careful after using it to wash it out of the hair)
5.) If your horse is really swishing his tail to combat flies then it is really a good idea to use some sort of tail bag. I used to make my own out of old worn out jeans but have also found that electrical tape, panty hose and tube socks work wonders. The modern day tail sock, though really stylish and pretty to look at are easily torn, ripped and don't protect the hair as well.
6.) If putting your horse in a bag or sock, you can also add strips of a tough material such as denim as a fly swatter. The main reason to do this is still allow the horse to protect himself from flies but protect his tail from the beating it takes when being swatted.
7.) Another really important thing is to make sure your horses tail is trimmed to no longer than his fetlock joint. If you make a blunt cut you will allow the hair to thicken over time. This will also minimize the horse backing up (either when riding or in the trailer) and stepping and pulling long sections of hair out. A thick tail always looks better than a long wispy tail.
8.) THIS IS SUPER IMPORTANT! Make sure when placing your tail bag,or sock that you are at least a good inch below the tip of the tail bone. If you wrap and tie that sock on any farther up, you run the risk of cutting off circulation to the tip of the tail bone. That is a disaster!
9.) To get those beautiful longer manes you will also have to do some maintenance. For horses that have lots of hair like Friesians or some of my reining horses, I typically keep the mane braided in long braids using sections of about 2-3 inches working down the neck of the horse. You will have to use small mane rubber bands and get them tight enough to hold the braid but not so tight they break the hair. If you can keep the mane carefully tangle free on some of the shorter mane horses you may not need to braid. I also have found that if you stop the braid on a thicker portion of the hair the rubber band doesn't tend to break the ends off as bad. You will have to be sure to replace braids and remove tangles (gently) often and may be washing the mane more like once per week.
10.) A word of caution about the tail covers that have you braid the tail into 3 braids and then put into a sleeve and braid the 3 braided sections together. These do stay on very well, but I have know a horse who while swatting caught his tail on something in his paddock and actually broke his tail so those may not be the best option. At least a sock, or tail bag is likely going to rip before it breaks the tail bone.
If I am trying to grown my horses mane or tail I am careful to diligently keep the hair clean, conditioned and be very careful when brushing it out, keep it up especially during fly season or hot, dry weather and never use a metal comb or brush in fact I usually use a brush similar to a human bristle brush. Those metal rakes just pull and rip the hair out and are disastrous to the hair.
Remember if you find your horse is stepping on pulling his tail out, it is likely just a little too long. If you are showing your horse and a super long tail is desirable be careful to keep it up when you are working it so it doesn't back up and step on the tail.
Good luck growing those luscious locks!