Alot of brushing. I use regular shampoo to keep my mostly white tricolor clean. I give her a bath the day before the show, the wake up early that morning and bathe her as well.
She dosent really stain, shes never had grass marks, just some poopie stains on her butt. They usually just brush right out after they are dry. The secret is to keep them as clean as possible by brushing as much as you can on problem areas. I dont like to bathe my horses more then once a week.