Lucky Braids shampoo and whitener. I cannot stand the purple stuff… it dries out the hair. I worked really hard to maintain the grey horse I ride and another rider used Quick Silver on him all the time this summer and now his tail is like straw, yellow, and breaking:(
My routine for show bathing: spray the whitener in tail (generously) and on trouble spots like elbows and stifle area. Then I ride. During the post ride bath, I shampoo the tail first and really work the hair to help release the stains with the shampoo. I then rinse the tail and saturate it in the whitener.
I bath the rest of him, holding a sponge and squeezing the soapy water on his body while using a pimple mitt to work it into the coat. I also use the pimple mitt while rinsing to make sure all the suds are out.
Then I rinse the tail. When I first started this routine I would saturate the tail again in whitener. After a while I could skip that last step as his tail whitened pretty easily with just the shampoo.
I also curry the heck out of him daily.