Since I have shown and worked with Walking horses and sadllebreds (and just love long thick manes and tails) here are some of the tricks of trade I have used off and on. I have used a plethera of suff but I will list only the ones I have seen the most results from. Like what has been stated above a good balanced diet (and teeth) are the first key to good coat and manes and tails. Not to mention good grooming tactics (I go through alot of rubber curries for the coat, lots of elbow greese)
Don't over comb (begin at the bottom of the mane/tail and work towards the top/crest line or dock of tail) and never put a brush to manes or tails. I pick tails and I stay away from all silicone products. (I do use a little Show Sheen on show day but its washed out and conditioned the day after show) Otherwise Cowboy magic detangler for realy tangled up manes and or tails
Supplements : I have used in the past Flaxseed oil (called Linseed oil which is made from the flax seed.) Corn oil (use about 1.5 to 2 ounces per 100 lbs of body weight a day. The same with Vegetable oil. I have tried using Clovite with some success, Calf manna, (good for general vitamin boost for working animals and showing animals) and Dapples which I didnt see any more results than with the aboved mention oils.
Topical applications: Mane and Tail is one of my all time favorites (for growth and condition): I wash with a gentle soap (usualy baby shampoo or Mane and aTale shampoo) then I squeeze it damp with a towel. While damp I apply generous amounts of Mane and tail Conditioner, work it well into the roots (the roots must be clean from dirt, dead skin and etc) and then let set or dry. After that I will braid the mane in stable braids (about 1.5 inch sections) begining rather loose at the top and tighten as I go down and then tie off with rubber bands. (Mineral based oils will eat rubber bands and I havent seen much lasting effects come from straight up mineral oil based products anyways.) With the tail you do the same thing. I change the braids only if they are getting messy.
I used Cholesterol (in the ethnic section of any department wtore like Wal Mart of Kroger or KMart) once a month and leave in about 3 days then wash out. The only draw back is that it will collect dust and does leave a waxy residue but does put a nice deep conditioning on it.
Stinky but does work is MTG. Esp on slow to grow manes, itchy tail docks or itchy mane crest. What ever is in it it does not like rubber bands and thus needs to be used with laytex bands or the mane left free flowing. Use it on the roots only for it will work down the lenght of the mane or tail on its own.
I will use a clarifying shampoo (human strength) every now and then to strip the hair shafts of built up dirt, oils, residue conditioners and etc. then recondition as staed above with Mane and Tail. I think Cowboy Majic has a clarifying Shampoo out there now. I'm cheap so I use Suave brand. The other key is to get the conditioner near the hair roots and message into the skin. Tea Tree oil champoos usualy do a good job of realy cleaning the hair. (I use it to help control my seborrhea dermatitis during winter months)
1 application that I found on accident was the Avon Skin So Soft fly repellent recipe. I used to use that frequently and when I used it I would spray a little on the mane and after some time I notice the mane was getting soft almost like healthy human hair. I was suprised at this. It felt so nice, didnt seem to do much as far as growth was concerned but texture was better. I don't remember the recipe right off hand but I am sure someone on this forum has one.
Anyting with that Jojoba crud in it I give it a big fat rasberry. Useless in my opinion. Another marketing gimmick.
Last but not least........Time.
Some manes a just genetically thin and don't grow at fast paces but with the above it can be helped along.