Bathe the horse the day before the show with regular shampoo...I would suggest Dawn's Dish Soap, especially for cleaning sheaths/udders. I use Martha's Blueing for manes and tails and white markings (but not on the face). Make sure you wash it out quickly, or you could end up with a blue/green pony. When you're finished, scrape all the water away and spray everywhere with ShowSheen unless your saddle is prone to slipping, then leave the saddle area out. Brush it into the wet coat and then let the horse dry completely. You'll have to hand walk or graze them so they don't get anything on the coat until it's dry. Put a sheet on if you can but if you can't, the show sheen makes any dirt super easy to brush out the next day.
Mix up a bottle of Wisk and water for stain removal at the show and take along the blueing. It could come in handy. xD Good luck!