I know this is going to be un-popular but:
Diet CAN help. It isn't the be-all-end-all for what Mother Nature gave the horse in the first place but diet can help.
My 25 yr old with equine metabolic syndrome has always had a "B" in the mane department. It almost dipped to a "D" when the EMS exploded on him.
Last year he started having frequent colics until I figured out he can no longer eat more than 1 to 1-1/2 pounds of "stuff" in his feed pan anymore.
Along with feeding him three times a day, I switched him to a high dose liquid vitamin that only requires he eat one ounce daily
Since starting him on the liquid vitamin last October, his mane has gotten a bit thicker and has grown a couple more inches. It's a little wavy and not-at-all fly away.
His tail always did drag the ground but now it drags the ground by close to a foot. I don't keep it wrapped, so that means he's out in pasture dragging it all over the place without it breaking off
Making sure the horse is worm-free is also critical to a healthy coat/mane
The only real fix to obtain a truly luxurious mane that wasn't born into beauty, is to buy extensions if the horse is going into the ring