There is a shampoo out there called BioGroom (good luck finding it, I haven't seen it in at least 2 years), they ahve a website (http://www.biogroom.com/), but I can't find anyone around that actually sells it. Anyway, if you can get this stuff, scrub it in the tail and let sit for about 20 minutes. Come back and rinse out, repeat cycle about 2 times and you will have a clean tail.
To get the tail to STAY clean after you have already shampood, try 4 lemons juiced (toss some of the peel into a spray bottle) with water and just a tad bit showsheen. Spray this on twice a day (don't drench the tail, just a light does like you would the day of the horse show with show sheen), come through and toss the horse in the pasture. DO NOT WRAP THE TAIL, it will defeat the purpose of the lemon juice. By a week you should notice that the tail is almost translucient (sp) due to the acid in the lemon breaking down the dirt. This is the ONLY thing I have found that works 100% of the time. No matter what I do, I wash the tail weekly and apply this spray, it really works.
For other shampoos, Jeffer's makes a whitening shampoo that works pretty darn well, you just really have to scrub a few times for everything to come out. Cowboy magic has a decent shampoo again, if you don't mind scrubbin a heck of a lot. Quick silver dried out my guys tail and caused some really nasty broken ends, so I haven't used it since. Years ago, I remember someone who was in (still maybe in, haven't seen her in a while) the TN walker show circuit, once the season was over she bagged her horses tail (white horse) and sprayed with showsheen, coated the whole tail in Mane and Tail Conditioner and left it wrapped for over a month. When she took it out, WOW gorgeous long, full and softest tail! I wouldn't do it, simply b/c of the fly problem here, but her horse was stalled with a fly system.