I have the Cowboy Magic Green Spot Remover for my horse (specifically for his knees/hocks... he actually manages to stay pretty clean everywhere else) I find that it's good as long as I use it regularly. If I wait until he has bad stains and then spray him, it doesn't seem to help at all.
When the weather is warm, a rinse off with one of the higher pressure settings on the hose (not too high... make sure it's still comfortable for the horse) does a good job of getting up under the hair to wash out the dirt.
Someone posted this brush
in a thread recently; I'd love to give it a try. Not sure how DH would feel about my 'borrowing' his drill though