Lots and lots of currying and grooming to bring the natural oils to the surface of the coat- it helps repel dirt! Vetrolin White N' Brite works great, but I wouldn't bathe more than once a week. If you need some really intense deep cleaning, Orvus paste shampoo is awesome.
Purple shampoo/dry shampoo, Lycra hoods, chalk and mud repellent!
You could use Yellow out, I have a appaloosa and it works Wonders on her white marks!