Quiksilver works great, but can get really expensive if you have to use it a LOT.
Since I have a mostly white horse who loves his green manure stains, I cheat. I order this: Kelco Plum White Shampoo - 1 Gallon: The Shampoo Lady
Yes it's a dog shampoo but it is also marked on the bottle for horses, cats, ferrets etc.
It's expensive, yes, but for one gallon at $32.55 you need to remember that it dilutes 50:1. So in the long run it is VERY cost effective, and it works better than anything I have ever tried.
My ritual is this - bathe Claymore with the Plum White, really scrubbing and letting it soak in. Rinse well and dry - once dry I coat him down heavy with show sheen head to toe. They helps keep off the staining and most dirt will just brush right off, for a little while at least. He still tends to get bathed once a month in the summer months. But this helps the "white periods" last as long as possible!