I like to do a lot of flexing, side to side, which increases the vertical suppleness of the horse. From there I will hold and release, aided by my leg. I'll hold, push, and when the horse "dips", I'll give a large release. During these hold and release sessions, I will lighten my touch incrementally. I want to be able to either squeeze or just pick up my hands for that dip. This continues for several months until I will expect the horse to hold his own head set. I will place the hold, then release and expect the horse to keep carrying himself. If he drops out, below or above, I'll take one rein and bump him back into place. Not a large bump, but just a little reminder to stay. I hate to say that I do a bit of rolkur, but sometimes I will overflex, hold for a couple strides, and then let the horse continue in a natural way. The horse will keep its head pretty natural for me so long as I've engaged the hindquarters and core muscles. Not to mention, my horses are pretty much bred with the even/low headset. After that I try to get my horses used to doing more complicated manuevers on a loose rein. Like if I wanted to teach the leg yield, I would start out with the give and release, and after a while simply correct by pulling on the inside rein. I've also learned that if the horse departs from a transition with a fixed head set, he will be more likely to keep it than if he departed unbalance. Blah blah blah, lol. It's all about feel and sensitizing the horse to the bit, and allowing him to be responsible for himself.