So I was riding my gelding tonight that I said was a lot like Zierra in the bracing and evading business. Tonight he was soooo heavy and was completely refusing to do trot/halt and canter/halt transitions. Even though we just did them last time fine. I had to unlock his jaw, neck and shoulders to get a supple halt. To do this I started playing follow the nose in what I call the drunk run. Where you take a few steps left, then straighten, then go a few steps right. Back and forth at the walk and trot. This gets them following their nose and working off one rein like Kevin said. Then I started alternating my half halts. I would bump with my left leg and collect with my right rein, then the next step I would bump with my right leg and collect with my left rein. I was basically bumping him up into the bridle but alternating sides so he didn't have a steady pressure to brace against. I got some really nice supple halts out of this at the trot and then called it quits for the night. This may be something you could do even in the hackamore on the side of the road to get her to sit on her haunches and raise her front end a little.
On a side note, thanks for reminding me about following the nose Kevin. I tend to forget about the simple things when I get into fights with my horse about how he can do something one minute and then shut down the next. It helps him regroup physically and mentally.