TrailHorseRider, I hate mud in the corral! We put down geo-fabric and six inches of pea gravel. No mud. We have to scoop two or three times a day or we'd be knee deep in manure, but that's the tradeoff. We are getting quite a bit of manure build up in the gravel, but even where it's really getting bad there's no mud.
Project for this summer: figure out how to wash the manure out of the gravel without using too much water. A sluice would work great--but it'd take way more water than we have. Any ideas, anyone? I'm thinking of "wet sifting" in a trough: put the pea gravel in a screen bottom box and lift it up and down in a trough of water a few times to flush the crud out of it. Be a lot of work, and take a lot of time, but wouldn't cost much.
And the left over water would be wonderful on the hay!