Parelli teaches it like this. Have a little riding crop in your "neck rein" hand. As you apply pressure on his neck with the rein, the riding crop should move up beside his face (don't wack him accidently) and just sort of jiggle it there beside his eye. His natural reaction will be to turn his head away from it. And you know how it is, where the horses head goes, so goes his body. The minute his head turns away from the crop, drop it out of his line of vision and ease up on the neck pressure. Move him up a few steps then do it again. As he gets used to turning his head away, you can begin to maintain the pressure, both on his neck and at his face level so that he actually is turning a couple steps. After a while you will begin to be able to just use the rein as it starts to click in his brain.
I think that by using just one rein, you maybe also train yourself and get away from the habit of two handed reining.