Aside from keeping her all wrapped up and trimming her tail, your horse will always be dirty because of the color. Horses roll. If you are looking for shampoo ideas, I use dawn with bleach (or the generic version of it) on any chrome. It works really well.
Personally, I wouldn't trim the tail, but I would definitely put it up. That's the single easiest way to keep a horse's tail clean and can be done for any time of the year. As for the mane, I wouldn't worry until it comes show time. Posted via Mobile Device
They do sell whitening shampoos, just leave it in for awhile before rinsing. I also remember someone talking about Goop. I think that is how it's spelled. Anyone else remember that?
I was one of them! Goop is inexpensive and works VERY well. It's a mechanics hand cleaner sold in the car wash area of wlamart/target etc in a white tub. You use it like a conditioner and leave it on for a minute or two and rinse. I liked the whitening shampoos too.
Bleach is toxic and not good for the horse at all. Would you want bleach on your skin, it drys badly. Stick with the shampoos made for white and the tail bag. You could use an all over body slinky but a horse needs to be a horse, not a Barbie doll. Also when you are going to the show you can use baking powder. Pack it on thick then lightly brush it. Enough of it will stick.