I've always used oversized lightweight sundresses over my breeches and show shirt to keep myself clean without making myself any hotter. Sure, I look a little silly swimming in some huge flowery mumu thing, but it's comfy, and I stay clean and cool.
I keep my breeches clean at shows by wearing my Nike windbreaker style workout pants-- I got them from an athletic store off of the clearance rack so they were a great deal! I find I don't get too hot in them and they keep me clean. Good luck!
I have athletic pants that I bought from WalMart. They are light weight and they have snaps down the leg, so they are easy to remove over your boots. I get a larger size than what I would wear so that they slip on easily and aren't so hot.
I also use scotchguard. It doesn't keep them from getting dirty but helps the dirt to come out when they are washed.
I'd love to try the new breeches that aren't supposed to get dirty!! Has anyone tried them?