My horse has a long, thick mane that gets longer as it approaches his shoulder (and is well past the point of his shoulder in length, nearing his knees, so a whole lot of mane).
Beginning behind his ears, I do a running braid that is not right up against the crest at the start, and gradually gets further away from his crest as it nears his shoulder, so that the hair length between crest and braid widens as the neck widens. In other words, when seen from the side, the braided mane is longer in length near the withers.
I find that not braiding tight to my horse’s crest helps the braid remain in place; think it’s because he can move his neck without putting tension on the braid. I finish the braid near the withers with a braid tail, which I partially roll up (to be closer in length to the nearby mane) and rubber band.
I’ve tried braiding tight to the crest, small and large sections, Quik Braid product, and dividing his mane in two along the crest, etc., without much lasting success.
I do find that the braid seems to stay in better when his hair is not super clean and soft – a little bit of dust/grime seems to give a bit of grip that holds the braid in place better, as compared to his freshly shampooed/conditioned mane that looks gorgeous and feels soft, but is too slippery to hold the braid well.
Good luck!