Lots of good suggestions...I am going to try orvus, never heard of it before.
When I bath them a lot (summer time), I use liquid ivory dish soap...a few DROPS in the bucket to get it suddsy. It wets the water and leaves no residue w a good rinse....its economical, and I feel there is nothing to gain w more expensive "horse shampoo". But, I also like cowboy magic shampoo, that is actually pretty good used sparingly.
For touch up I am impressed with Miracle Groom for spot remover. It smells nice, and wow! It takes the spots right out!