Well, conditioner will help it a lot. I like using a (people) conditioner called Fructis Sleek and Shine leave-in conditioner. It's works great. My horse's mane is about six inches longer than it was when I got her, and her tail is almost a foot longer. I know it helped because before I started using it her hair didn't grow near as fast. I bang her tail every now and then, also, to keep the split ends from moving up.
I also use this mane comb called a Grooma Rake. Here's a link to it: Grooma Rake - Grooming Tools from SmartPak Equine
I can swear by this thing. I love it! It has spinning teeth so it's doesn't pull out hair, and it lasts. When it used to take me at least 45 minutes to brush out her mane, tail, and forelock, it now takes me 10-15 minutes. Brandy's mane is over two feet long, and her tail would drag the ground if I don't cut it, so that gives you an idea of how long it would take to brush! (I always put conditioner in her hair before I comb, btw.)
I recently started putting maintenance braids in my horse's mane and it's growing even more. I take it out every week and then condition and re-braid. So maybe you could try that, too.