The big thing to remember is that the halt and reverse are both FORWARD
movements. This cannot ever be overemphasized. Also, there should never be a pull on the reins.
For a halt, give a deep exhale, still your seat - stop following the motion of the gait, close leg, melt into the tack (as Snookeys put it, make yourself 50 lbs heavier), and close hand. Not a pull here, just a cessation of the following motion of the reins if you're riding with contact, or a slight lift of the hand if you're neckreining. You're just closing the door forward. Don't slam it in her face with a pull, just gently close it in front of her.
To back up, again think of it as a forward movement. My big problem was that I was looking down, collapsing my chest. Look up, imagine that your ribs are a sail filling with wind, create energy with the seat and leg as though you are going forward, but block that positive motion with another gently closed door from the hands. I also tilt my pelvis slightly forward from the seat bones, farther from my pockets. I'm not sure if that tilt bit is totally kosher classical form-wise, but it seems to help Scout.
To illustrate what pulling does, think about that overused action-movie problem of the room with the walls closing in. That is stressful, claustrophobia-inducing, etc. Now think about just approaching a stationary wall. You know it's there, it isn't going anywhere, and you can't walk through it. The only other option is to back away if you want to move.
Good luck, and enjoy your NATRC ride!