I have some powder stuff which cost me $3 works wonders, after reading this I think I just got baking soda with a posh label lol I can't remember the brand but it has a gold label and comes in a hand sized tub and it's just over a year with more than half left :) and that is washing tail most weekends during summer and every 2/3 weekends in winter
She isnt very clean, but her tail is!
You could try Woolite. Its not harmful to the horse, not even if you don't rinse it all off. It helps keep dirt and things from coloring her tail and mane (also if your horse has them feathers). Another thing is that it keeps your horse feeling extremely soft for like 2 weeks. I've tried it and it works wonders with my flaxen maned sorrel.
I use bluing, corn starch and simple green. :) I also use downy clothes washing detergent to keep my horse's mane and tail tangles free. (use it as a wash first and than apply more for it to soak into the hair. Don't use on the mane if you want to braid!) It also gets all that useless dirt out of your horses mane.