The main secret to long manes and tails..... GENETICS! haha. If your horse doesn't have a good hair gene, you're never gonna get a dramatic mane.
But secondly they are kept braided and wrapped up and they never brush them. If you notice their manes are all wavy, that's because they were just taken out of their braids. I know of a few reiners that when they wrap up manes they wrap the hair around little 1 or 2 pound weights because it gently pulls at the root of the hair and kinda forces it to grow.
What we do for long manes/tails is we NEVER brush them, unless they are about to go into the show ring, then we only finger pick through them. Combs never touch their hair. They get taken down and washed before shows and in the very beginning of summer, that's it. Some of their tails will stay untouched for a couple months at a time, there's no problem with that. Some of ours that don't have good hair genes get the help of MTG about twice a month. We also put a leave in conditioner like Infusium 23 once a week and get a whole body spray of Healthy Hair Care about every day.