Wet the tail well with warm water, then wash it with your favorite shampoo.  The secret to getting tails clean is to wet every hair, which is not as easy as it sounds, especially with thick tails.  I find that Horse Spa Tail Buckets are a fantastic tool for getting tails really clean.  For a white tail, after shampooing and dumping the dirty water out of the tail bag, try putting some whitener back into the bag with a little more water and tying it back up. Let it soak while you work on the rest of the horse. 

The soaking each hair applies to the mane as well.  I like to use a mounting block to get up and over the top of the neck.  This allows you to wash the mane right down to the roots.

When bathing the body, I choose a shampoo that is safe to use often.  Vetrolin the Stübben Hair Care Line are two top quality shampoos.  Wet the body with warm water, and use them as directed. For especially dirty spots on the body, like grass stains or manure stains, try putting shampoo directly onto a Grooma Soft Touch Massage Brush and work the lather into the stain.  Let it sit while you work on the rest of the body.  This works well with general problem areas as well, like hocks and knees.  

Don't forget to check the sheath or teat area! 

Many horses have white legs, and in my case, feathers!  For the sake of your back, train your horse to stand quietly while you pull up a stool and sit to work on his legs.  I have one of those little garden stools with wheels that works  great.  Be VERY CAREFUL when you do this, even the most well trained horses can surprise you with a sudden move.

After you thoroughly rinse the body and legs, be sure to scrape off the excess water with a scraper.  This is important because leaving water on the horse's body creates insulation and will not allow heat to escape.  If you don't scrape off excess water your horse can overheat quickly in hot summer weather.  Rinse and scrape both sides several times until you feel sure that all the soap has been rinsed out.

A quick tip for the next morning when you find dirty spots on your show ready horse, or when you unload him from the trailer and find manure stains on his clean tail and legs, give the Shapley's Easy Out No Rinse Shampoo a try!

Now back to the tail, which has been soaking.  After taking off the tail bag and dumping the whitener, you can leave the tail to dry if the whitener you use allows for that.  Be sure to check the label to be sure the product you use does not need to be rinsed out.  You can also condition the mane and tail at this time if you like.  Again, check the labels for directions.   Some whitener products can be sprayed on a wet or dry tail and left on without rinsing.

The face is the last thing.  Be very, very careful!  Some horses just won't stand for hosing the face; it can be a very dangerous thing for both horse and human.  Use a shampoo that's safe for the eyes, even human baby shampoo is a good choice.  If the horse will not tolerate hosing the face, don't push the issue.  Wash his face with a washcloth and towel instead.  If he allows you to rinse his face, a bar of baby soap is an easy way to wash his face and not get soap in his eyes.