I have an all white horse so I have a few tricks up my sleeve. I got a concentrated blueing shampoo (REALLY concentrated) which I dilute for his body, but I just put it directly on his tail/mane and let it sit for about 5 minutes before rinsing off. I don't remember the brand, I'll check next time I'm at the barn.
Then to keep the tail white, I use a leave in whitener (should be able to find at TSC, it's in a spray bottle) and show sheen on the tail, braid it and wrap it (with vet wrap for long term, like over the winter, or in a tail bag if it's a day before a show). I usually don't have as many problems with the mane, but if it's stained after I wash it with a blueing shampoo I'll use the leave in whitener as well.
Take a ride across the badlands
Feel that freedom on your face - Breathe in all that open space
You'll understand why God made -Those fly over states