I can't say I have a particular way on how to train for it but there are two mistakes I see with greener riders.1. When they neck rein they drag their hand so far over the horses neck that the outside rein is being pulled, which just cause the horse to suck back. I think that's also why I see so many riders with horses that root their noses out whenever they are asked to turn.
2. In the training process a lot of riders never pay attention the horses nose, nose FIRST, not shoulders, not rib cage. I see it all the time, the horse is just throwing his shoulder into every turn.
As for number 2, I disagree slightly. A horse's body position IS very key in neck reining. If you concentrate on JUST the nose, the shoulder and rib cages bulges out and the horse is merely turning it's head and neck. You want the horse's body to follow through the turn, which is why paying attention to the shoulder and rib cage is very important.