With white horses, first I use Suave shampoo. SCRUB SCRUB SCRUB! Take your time and get in deep. It's cheap and does a great job of cleaning (which whitening shampoo doesn't always do. Plus, it's expensive!). Rinse off the suave and then then I take a good whitening shampoo (I like Quiksilver), slather it on, and leave it to do it's job a good 15 min or so. That's what really makes them glow. I hang on to them while they're drying so they won't roll and then spray some Show Sheen onto their tail and legs to repel dust. Some of the really stained horses will take more then 1 bath to really get white. Might depend on the horse, but i've never dried out a horse's skin.