Ok, I know this sounds crazy, (it probably is!) but it's worked really well.
In a spray-bottle, mix equal parts water, white vinegar, and conditioner. Shake well, spray on tail, and brush out as gently but efficiently as possible. I've started doing this every other day on my horse's tail, and it's working nicely. (Though the regular brushing probably has a lot to do with it)
This was a recipe from this month's Horse & Rider magazine.