I believe the best way to create outer health, is by starting on the inside. Hair you see is already dead, the only living aspect of hair is the root and follicle. If those aren't healthy, the hair they produce won't be either.
There are quite a few supplements you can give to help hair growth and improve coat quality, the best ones are Biotin, which is an essential B vitamin, and vitamin E, which contains polyunsaturated fats that are shown to have a wide range of beneficial qualities, including improved skin and hoof quality and joint suppleness.
One of the best natural sources for these vitamins and fats is flax seed, especially flax seed oil. One or two tablespoons mixed into the feed once a day is enough to be beneficial. Giving too much can cause diarrhea due to expulsion of the excess oil, but otherwise, it is very safe.