The key to healthy hair is good nutrition! Make sure your horse is getting all the vitamins, minerals, and other nutrients she needs in her normal feeding regimen.
After improving diet, really make sure to take the time daily to work with her mane and tail. If you keep up on the maintenance of it, you'll notice it will be easier to work with. I much prefer taking an extra 5-10 minutes daily then having to have one long session trying to fix the mane and tail up. I really like Ecolicious Equestrian products, especially their De-Stress treatment for the mane and tail. You can find it on their website here
. Not only does it smell great and work, it's good for the environment too and has no harsh chemicals in it!