I wouldn't bother trying to keep it 100% clean until you're right about to go to a show. If he's neat in his stall you can help keep him clean by making sure the area where he lays down is clean, and turn him out with a waterproof sheet/blanket when it's muddy. If he's not neat in his stall or really
enjoys rolling, you're fighting a losing battle
When you do bathe, I really like Orvus shampoo.