Rice bran pellets. I originally added this to the horse's diet to help with wieght gain but the change from his winter to summer coat is amazing. He is super smooth and shiny. This is a top dress additive but is limited to 8 cups per day, and, you would want to do a gradual buildup over a period of 4 weeks.
It's been said a million times already but brush, brush, brush. Use a rubber curry (look up zoom groom) or a natural bristle brush. If you do bathe use a shampoo with natural based cleaners like coconut. Not harsh chemical cleaners. An oatmeal shampoo will keep the skin from getting dried out.