Here's how we do it in the Paint and Pinto show world (and as you know... some of them have crazy-white legs!)
1. Under the knee and hock, the hair is clipped AGAINST the grain with a #10 blade. It looks fabulous, if I do say so myself.
Remember to do this about 5 days before the show, so the hair has time to settle if you leave any marks.
2. Horse is given a bath, and before drying off is sprayed with show sheen, to help prevent dirt. A lot of people use whitening shampoos, but I don't feel like you have too unless it's above the knee/hock.
3. Come show time, pop out a Shapley's can of white touch up spray! It's incredible. I have it in the black for my bay as well, and I can't say enough good about it.
Not a lot of work required and a really nice turn out. :)