After a lot of struggle trying to get Cassie to stop pulling on her leash, I finally gave up today and got her a prong collar. Dogs don't much like them, and neither do I, but no amount of treats or yanks on her harness worked with her.
As soon as I switched the lead from her harness to her prong collar, Cassie suddenly knew exactly where to walk on a heel, and only pulled enough to make the prong contract before falling right back to my side. We walked about 4 blocks total, and she never pulled enough that I needed to correct her. Amazing.
I realized after about a minute that she's worn a prong collar before. I worried that she would hate it, that we would have to spend a couple of weeks working with it for her to make the connection between pulling and neck irritation, but no. All that pulling for the last 10 months? The harness just doesn't provide enough correction.
In other words, she has always known how to heel, she just hasn't wanted to. Smart dog.