Unless there is a reason to have them spit shined (ie showing) I find it easy to keep them looking good through the winter with some good ol' fashioned elbow grease grooming. We actually don't bathe much at all, even when it's warm. Is there specific areas that are extra dirty on your pony or types of dirtiness that are a concern to you?
For winter lessons and shows, I resort to hot toweling and vacuuming. Keep a small section of your horse exposed and scrub it with a hot washcloth. Dry it, cover it with the blanket, and expose a new area. Posted via Mobile Device
We have a hot water wash rack. If the yare really bad I will do a warm water wash and immedietly cover them in a cooler and blanket... usually I do this with my mares before they foal. I like them to be clean and not have bacteria on them. Plus they seem to feel better :)
Now, where my horses live out and I don't have hot water in my barn, I wouldn't dream of bathing them.
Previously where I was at a fully equipped livery yard, I would choose a sunny day, bathe them using the hot water shower (in halves with the other half covered with a fleece if necessary), then towel dry, then stand under the heat lamp (yes - it was a fully equipped yard!), then put a dry cooler/fleece/stable rug on and feed lots of hay. Later at horsey lights out time my YO would take the coolers off and put on normal dry overnight stable rug.
Lots and lots of brushing. I start with a rubber Grooma Groomer or other rubber curry. Working it in circles to bring up the dirt. Brush with a stiff brush then do the Groomer again until no more dust rises. It just takes time and you have to contend with a not so clean horse in the winter. Just get the saddle area as clean as you can and don't worry about the rest. If I get ambitious I will use a waterless shampoo that I also use on my dogs. It's very dry here so dust is available in abundance. It seems sometimes it is never ending on horse butts. Of course all of mine love a dust bath :lol
Have you ever tried a grooming block? It's like a pumice stone about the size of a dish sponge. Heres a link to one: Slick 'N Easy Grooming Blocks Farnam Just make sure your horse is dry because they disintegrate if they get wet. Once you get him clean, blanket him to KEEP him clean! That's the trick.