I always get to the barn early to make sure that my horses stockings are white. I always get to the show super early and just do them there, I suggest Corn Starch, whitining shampoo, and a spray on whiteing it helps the best.
For the mane, Hair gel! Its my lifesaver and it helps to keep the braids or bands laying flat on the side of the horses head.
The night before I suggest braiding your horses tail (just a regular braid) starting from under the tail bone and continuing down till the end. It helps keeping dirt out of the beuftiful tail. At the showgrounds take the braids out and brush through, just make sure to not make the braids super tight or your going to have wavy tails.
I Hope this helps =)
Horses are my life, if theyre wasnt such a thing life would suck!