I never shampoo my horses, but I will spot shampoo for manure stains, etc. I rinse weekly in the summer, and not at all in the winter. Rinsing lifts the dirt and sweat off, but leaves the natural oils, which makes for healthier skin and hair. I do mix either Pantene Pro V conditioner or Infusium in a spray bottle at a ratio of 3-to-1 with water, and spray the mane and tail each time before grooming, allowing it to set and dry. I finger-comb manes and tails, and once a week I use a vent brush to be thorough. I never brush/comb manes or tails wet (increases breakage). Both of my horses look like they have mane and tail extensions, which they do not. If money is tight, I will use Suave conditioner with coconut, and cut it at the same ratio, but I get the best results with Pantene, and it is still by far cheaper then Mane-n-Tail, or Cowboy Magic. There is no build-up, since the horses are rinsed regularily.
"I don't own my horses, they own me. It is a relationship I have no plans of changing."