When I used to show WP, we cut the manes (GASP!)
LOL If a horse had a super thick mane, we pulled it and used banded it (besides, banding was super popular then and kept the mane from moving as much)
For med-thin manes we cut with scissors. Not even thinning shears. First, cut about an inch longer than you really want (2in your first time to allow room for error) horizontally, from ears to withers. Then, push the top layer of mane to the other side if possible, if not just be sure to not cut it in the next step- then I angle the scissors where the the blade is higher against the neck (a sort of "bevel" in the hair) and cut horizontally again down the length of the mane. This makes the hairs shorter that are closest to the neck, and you are only cutting half the thickness of the total mane. For thin manes, I just do this step through the whole mane, without splitting the thickness. This step lets the mane lay flatter.
Then I take my scissors almost vertical, slighty to the left and right (alternately) and make random snips about 1/4 in- 1in deep all down the mane. This breaks up that blunt look from cutting horizontal. I just do this over and over. This takes the rest of the extra inch you left in the first place. In thicker areas (usually up over the crest the mane grows thicker) you have to make more cuts than at the withers. This is your chance to even up the original cut you made if you left any section slightly longer.Just make more little snips. It makes a natural looking edge. It works pretty good and you don't have to painfully rip hair out of the skin -which the horses appreciate. 8)