Winter is actually the easiest time of the year for me. My horses wear blankets, so their bodies stay clean, and other than that it is only snow that can get on them. I do like to give them a good, deep currying at least every few days though, because wearing the blankets can make them VERY itchy.
I did try some stuff called Miracle Groom, which is supposed to clean without water. It worked pretty well for some areas where I could tell that they had laid in their poop, and it smelled really good. I cleaned my mare's whole rump with it (using a cloth and rubbing against the hair) because she was kind of dusty.