I kept hearing how I need to shove my horse forward and make her uncomfortable in the usual position so that she tries the "correct" one. Boo. She has done plenty of sports before, and has shown that she knows how to round. Problem is, making her uncomfortable isn't her style. She would scrunch up hollow and try to bear the discomfort of a thick outside rein/abnormal pressure/etc. paired with constant legging. Ugh. I started the more natural method we like, on the ground, minimal equipment, lunging with a roomy halter and 20ft tether. I would praise her and get excited about her sniffing the grass, and pretty soon she would experimentally lower her neck to a nice Quarter Horse level instead of a giraffe height. I would say Good Girl to let her have the choice of walking, or keeping the gait. She is a Paint from racing lines, so she loves the walk and the gallop as her favorite gaits; her most hind end engagement and use of topline can be seen at those two. So, I would wait until she was walking lively but with a relaxed neck, and then ask for trot. She would snap her head back up and go around looking nasty, but as soon as I saw her lower her head, I gave the verbal praise so she could walk, and pretty soon she realized that the same form in the walk was good to have in the trot; she wasn't made uncomfortable in the trot, all I did was keep her from lagging in pace, but she recognized one form was better than the other.
I was still struggling with getting this under saddle, though. All this talk about shoving her onward and yet restricting her with an outside rein wasn't getting us anywhere solid, so I thought... why shouldn't what I do on the lunge not work in the saddle? So, I worked her for an hour as usual in whatever frame I could best get her in, and at the end she routinely stretched out and relaxed for the cool-down at a walk. I waited for the best moment as on the ground, and asked for trot verbally. She picked it up, went giraffe for a bit, but as I simply kept posting lightly and focused on having the softest hands possible while still having contact, she finally started lowering her head, and after a few times of Good Girl, she very well got the idea.
I put her in a paddock for free-lunging afterward in a Neck Stretcher set loose, good goodness her hind end was springing/swinging and she had her neck so lovely, anyone could tell she was liking her newfound stretch in trot.
That's it for you then---- have your trainers, of course, but remember to ride and train your horses with your own style. My girl already could move off a light leg aid spritely, so I do have the warning that anyone attempting headset and back rounding should have stop and go under control and as light as can be.