I recently asked my group of horse owning friends on Facebook, Instagram, and in person what their go-to methods are for growing healthy and long manes and tails. I got a lot of helpful feedback! Below I'll share with you the most common supplements and tricks that have shown great results.
Su-Per Hoof: This is a supplement with both gelatin and biotin, as well as some other vitamins.
BioMane: I personally would try the first three supplements before trying this one. But I've heard lots of great reviews from BioMane users! I would just rather test the less expensive options first before spending a dollar a day hoping it works on my horse.
Listerine Original (the brown one) + baby oil 50/50: The Listerine will help kill fungus and dandruff. The baby oil keeps the skin moisturized. I'd recommend doing this mix every time you redo their tail bag/wrap.
Ultra Nourishing Hair Moisturizer: An alternative to healthy hair. Use both (not at the same time. Remember science class? In order to conduct an accurate experiment you must have only one independent variable! Come on) and see which works better for you!
Bags & Wraps:
Tail Boot: This is such an awesome innovation and it's shown some really great results! If you don't want to spend the money on one of these then keep reading to see how I do my own horse's tail in a similar wrap using twine and vet wrap.
DIY Tail Boot: Braid your horses tail, wrap it up similar to this demonstration by the Tail Boot company here. Then instead of applying the tail boot in that demonstration video, take some pieces of twine (about 3 feet long) and loop the twin into the tail at the bottom and then tie them in a knot to secure the pieces. Lastly, wrap the tail with vet wrap to hold everything together. The twine gives your horse something to swat the flies with. So in the winter the twine isn't needed, unless you just want to make your horse have a blue fake tail :)
Manes & Forelocks:
So the length, thickness, and texture of their mane and forelock really have a lot to do with their genetics (with tails too, but generally tails are easier to get growing even with those that aren't genetically gifted).
For long manes: If you keep your horse turned out for the most part, definitely keep their mane braided so it doesn't get windblown, tangled, witch knots, or torn out. Remember to redo these braids at least twice a month and add a moisturizer every time. If you keep your horse in a stall and only turned out occasionally, it's still a good idea to keep long manes braided just to be safe.
For short manes: just keep them short year round so they don't 1. look ugly and awkward, and 2. so it doesn't get long enough to get tangled or ripped out.
Forelocks: do not braid or band them unless you're going to a show (or if it is already long and full then just treat it like a part of their mane). Keep them moisturized, cross your fingers, and pray every night for growth.
The "Duh" solution:
Time: Give your horse time. Be consistent with caring for their manes, tails, coats, and everything about them and over time you will see improvement. My recommendation after reading all this is to not go out and buy everything on this list and use it all. I'd start with choosing 1 or 2 things from the list that make the most sense to you and how you want to care for your horse, then buy them and test them out. If you've been using the same product for over 6 months with no noticeable change, definitely switch it up and try something else on the list. Same as with people, not every horse is going to respond the same way to these products. You also need to do what is convenient for you; so if you don't normally grain your horse, maybe try one of the bags or wraps only first and see if that alone will help. Or if your horse doesn't have a very healthy looking coat then I would recommend starting with one of the supplements to help them from the inside out. Start experimenting and see what works!