They make conditioners like MTG, Cowboy magic etc. I don't know that it makes it grow but it makes their mane a lot more manageable. Its best to finger comb their manes. If they get a snarl which they find ways, I saturate it with some type of slick conditoner and then I treat it like I'm trying to unbraid the hair. Try not to take a comb or mane comb thru it, some horses are very prone to tangle and some like my sisters horse has this super straight mane. My horse's hair does this sort of rope looking thing where it starts to almost twist like a dread lock I guess. I just saturate it and start to unwind it and then finger comb it. All of my horses have long manes.
My comment would mirror Chandra1313.
It has been my experience that, unless a horse rubs or has an issue causing hair loss, their mane is what it is. We have 4 horses. 3 have manes that grow to about the lower part of the neck and 1 gelding that has a short, sparse mane, and that is just the way it is. Also, some breeds seem to ge gifted in the mane and tail department while others can be a bit short changed. Often we just can't fight Mother Nature.
But good luck...maybe someone will have a miracle solution here.
The best thing really is keeping it clean and detangled. I'm not a huge fan of braiding as you have to be really diligent that the braids aren't too tight, you take the out often and re do them, it's a lot of work. Also, don't take a hair brush to it every day, it will pull and break the hairs. Like it was mentioned above, finger pick out debry and tangles. I like to use just a very small amount of baby oil and work it in really well. If you don't work it in, that's where it stays greasy and will attract dirt. Oh and also avoid using detangler sprays for horses, it contains silicone which hardens the hair and causes breakage. That's all I know and works for me.