I own a grey... I don't know what the word "clean" means... but here is my pre-show ritual...
2 days before: Curry the crap out of him head to toe. Bathe him with a good shampoo, suds him up and then curry him with the suds. Rinse thoroughly. Suds him up with a whitening shampoo (quiksilver, cowboy magic, etc), curry with shampoo again. Let it stand twice as long as it says to on the bottle (my theory: it may burn off his skin but at least the dirt will go with it! J/k) Rinse. Show sheen him wet. Hand graze him until he is dry so he can't roll, show sheen again.
1 day before: Curry/brush to get excess dirt off (because of course he rolled, there was no way he could stay in until the show because he is a TB and would loose his cookies come the day of the show). Bathe with whitening shampoo again, let it stand twice as long as it says on the bottle. Rinse. Use a conditioner. Rinse. Show sheen wet, and then dry.
By this point he is usually sparkling, and the total hours of bath time rounds out to about 3 & 1/2 (yes, I'm serious).
I put a light sheet on him and, holding my breath and praying to god, I turn him out. If it's not a really hot night (assuming we're on night turn out now) I will put a hood on him, b/c he has a mohawk for a mane if it is pulled shorter than 6 inches).
The morning of the show I use Vetrolin green spot remover to remove the spots I can't get off by spot washing. And then I use Horse Paint to cover what won't come out. Yes, I'm serious.
I will get the name of the products, it is wonderful. Just like spray paint but safe for horses, and washes out. Comes in all colors, too! Covers up cuts, missing hair, dirt stains, etc.
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."