Show sheen actually dries out the hair if you use it long term/frequently. I don't use anything special, I keep my geldings tail braided and in a bag as soon as it starts to get muddy out, mud dries out hair as well. I will take it out at least once a week and do a quick finger combing, some Xtreme or cowboy magic detangler if it needs it, then put it back in the tail bag. If there is a stretch of decent weather in the winter I will let it stay down until the weather goes bad again. Now granted my gelding obviously has good genetics in the hair dept, but I have done the same exact thing with my other or friends horses with less than stellar genetics in the thick, luxurious tail department. I keep it cut to at least fetlock level.
I also feed about a half cup of freshly ground flax a day, it seems to promote maximum hair growth. I noticed a significant difference in hair growth since I've started feeding flax and free choice loose minerals.