Add sunflower seeds to there grain it will help get the maine shiny along with there cot so you will have a all ourond shine horse and rarely need to use show shine at shows.
Quality diet and daily care with an excellent groom
The answer is.... Only their grooms know the answer :)
Honestly, when I've been working as a groom I've been trying to keep my horses' manes clean at home. When they rolled in their own poop I washed them just after I noticed the dirt. Beside that I used sprays and conditioners designed specially for the mane (sorry but I don't remember the exact brand but I like using Carr&Day&Martin products) :)