my horses tail gets dirty too, but I make sure to brush it on a daily basis. I'll brush it and make sure theres no knots, and when I go to give him a bath I always grab a LOT of horse shampoo [mane and tail brand works just fine] and then I act like its human hair and wash it just like I do mine, conditioner and everything, it works GREAT.
Also, before a show, or just to put the natural oils back into his tail, use mayonnaise to condition it [don't wash it out for a few days] and braid it up and put it in a tail bag to make it look really nice and shiny! [if you're going to do this for a show, make sure you do it about a week before the show]
You can tell a gelding, you can ask a mare, but you must discuss it with a stallion.