There is a mare at my barn, and she ALWAYS knots up her mane. And it is about as long as Spirits mane. My B.O. Puts lots of conditioner in it, and brushes it everyday! It is a pain, but its the only thing that works!
Haha yeah... I have a mare that has the same problem. What I did was work through it with my fingers... OMG, it took forever... But then I braided it and I haven't left it unbraided since. Works AWESOME. It is a pain to have to go through it at first, but still, well worth it.
And... WOW. B-E-A-Utiful! And so clean! Mine's always dirty... >.< haha.