I think keeping the mane well pulled and braided is the best way. My horse's mane naturally lies on the left; when I bought her, I pulled her mane and started braiding it over to the right. The lower half of her mane (starting with the withers) is now perfectly trained to stay on the right; the upper part I have to braid a couple of times a week to get it to stay.
I heard a neigh. Oh, such a brisk and melodious neigh as that was! My very heart leaped with delight at the sound. --Nathaniel Hawthorne
When I cared where his mane went, I would wet it with a damp brush, brush it over and lay a towel on it while I groomed pre/post ride.
Now that he's well into retirement he's a wild man and lovin' it, though I do still keep it short-ish. Some old habits die hard.
Not that it helped much, he has a super thin section at the poll including a forelock that is so wiry and fine and would break instead of pull out, the next third flops to the wrong side and is SO thick, and the next third is very thin again and he was so sensitive about pulling it near his withers. I learned to get very good with thinning scissors, which is what I use to keep it trimmed now, cause why rile the geezer up?!