When I was in Pony Club they had us use Listerine (the brown kind), just put it in a spray bottle, spray the roots and massage it in. I dilute it 1:1 with water.
I've also had luck using head and shoulders shampoo on horses. And some people swear by healthy hair care moisturizer: Healthy Haircare Moisturizer - Dover Saddlery.
Hope it helps!