Using MTG on the roots of the mane and tail really works for hair growth. I rode an appaloosa one year who went from wisps of of mane, so a normal mane in 2-3 months. If your horse already has a decent mane, keep it clean, detangled, and don't brush it often. What some people do is the mane in braids or in mane bags to prevent breakage. Toadstool Farm Mane Bags
Also, if you decide to keep it in braids/mane bags in the summer, be sure to be very diligent about fly control and grooming to prevent the horse from itching and rubbing the mane off.