Ewww, I knew a horse with a mane like that. I'm not sure how much of it was that his mane was really dirty and how much of it was just the way his hair was. His mane was so bad that you'd brush one section and as soon as you stopped brushing it, bam! It was amazingly tangly again.
I'd agree with conditioning the heck out of it. I might even use human conditioner and leave it on for five minutes or so, then rinse it off. I'm also a huge fan of Cowboy Magic detangler. I've been using that once or twice a week, very minimally, on Lacey's tail for the last few months and oh man! Lacey's tail used to be easily tangled and kinda rough to the touch but now it's SO soft and really nice to touch. It's also extremely strong and VERY healthy looking.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.