The trick to keeping a high gloss on your horse's coat is a series of simple steps. The first step is to feed your horse well.
Water, while not exactly a feed, is the first requirement. Horses, depending on size, activity level, weather and type of feed can consume anywhere from two to twenty gallons of water a day. If you find the water bucket empty on a regular basis you need to supply either a bigger bucket or a second one to insure your horse has an adequate supply of water.
The next most important item in any horses diet is salt. A source of salt should be available at all times. As an animal that consumes vegetable matter a horse needs extra salt.
Once you have supplied these essentials hay is the next critical feed. A minimum of one to two pounds of hay per hundred pounds of horse is necessary for optimum health. As grazing animals they need a continuous source of fiber which is supplied by various grasses and woody products, such as the leaves and twigs off trees.
Grain should be kept to a minimum. Only horses working an hour or more a day should be fed grain, but in an environment where everybody else gets grain this is hard to do. Therefore a quarter pound of grain may replace a pound of hay with no more than a pound of grain per feeding.
Next, make certain your gets adequate exercise. Horses are animal designed to move constantly and being kept in a stall with no exercise will lead to more problems than merely a dull coat. I am not talking about endurance levels of work here. A twenty minute turn out, once a day, will do wonders to help a horse maintain its mental stability. A couple of hours would be better and constant access to a pen big enough to trot around in is best of all.
Now for the big secret to maintaining a high gloss coat on your horse NEVER, EVER use shampoo. That is unless your horse has a run in with a skunk or some such disaster. Instead, work you horse until it has at least a hint of sweat, then hose it off until the water runs clear. This gets the dirt out of the coat, but does not remove the oils that give that great shine. Scrap excess water off, walk until dry, and your horse will look as if you slaved for hours grooming it.
Ah yes, if you are fortunate enough to have access to hot water be sure and keep it at or slightly below body temperature. Try it on your wrist as you would a baby's bottle. Water that is too hot will strip valuable oils from your horse coat.