It takes time, but what you're doing works.  My haflinger/belgian is not too keen on picking up subtle cues, but one day I just found that she started to get it. It was like a lightbulb came on.  I can pick the spot on the trail that she had her revalation.  I've also crossed the reins under their chin and rode that way.  I use a myler bit with independent side movement when I ride like that.  When the neck rein goes, say on the left side of the neck, the horse first feels the rein, then the familiar pull from the right side of the mouth.  It worked for me, but I'm sure there are some people who will object to that method. I felt that it helped me be more consistent with my timing and signal.  I practiced a lot in the arena asking for turns around cones/barrels. Whatever you do, don't get the "onehander" bit.  It doesn't work and confuses the heck out of a horse. 

Amanda