I would literally saturate the entire mane in a conditioner and then very carefully comb it out, starting from the bottom, of course. I am a big fan of Fructis Sleek-and-Shine leave in conditioner. It is a people conditioner and it works really well. I put it on my horse's mane and tail religiously. And I love the Grooma Rake for a mane/tail comb. It has rotating teeth and does not tear the hair out. It is sooo worth getting. I can brush out my horse's mane, tail, and forelock (which are all very long and thick), in about 8-10 minutes with it, whereas it used to take me closer to 20-30 minutes!
Here's a link to it: Grooma Rake - Grooming Tools from SmartPak Equine