I really like the Tangle Wrangler... It looks like a hairbrush but is in fact several combs "stuck" together at the handle. I don't know why, but it really makes short work of tangly manes and tails.
Also, for really thick manes (my gelding's mane is 2 inches wide, I swear!) I sometimes use a dog de-matting brush, which is really several sharp blades that you can drag through the underside of the mane to remove a bunch of hair without leaving bald spots... Probably not orthodox, but it helps!
I have seen horses whose crests have "fallen," apparently because of the weight of too-thick or too-long manes. So I keep my beasties trimmed and thinned, as time permits, of course.
Hope this helps!