Between all the good NH trainers I've seen, there are some things in common, and I'd like to also point out some fairly successful methods, and some of you can probably think of more;
Spend time observing the horse. Without trying to get something out of the horse or wanting to interact with her, sit back and watch. Much can be learned simply by looking at what the horses do throughout the day.
Have a few photos on hand, or better, videos of the horse before, during, and after your training sessions. Is she stressed before and relaxed after?
Know horse and person body language! Read up on it, watch videos, observe your own horse, etc.
Don't be afraid to experiment creatively somewhat. Within safety, test the waters; change your stance, be less active or more active and see what happens.
Above all, try to increase your awareness. The critical component of handling horses is to have a "feel", and this cannot be taught, though awareness greatly helps. Ever so often, check yourself; are you tense? Are you lagging behind? Do you just want to get over the session? Horses are sensitive to moods and mindsets... they have a good idea when you are half-hearted or frustrated.
Furthermore, if you've gotten to a fairly good spot in your relationship, practice carrot stretches: Stretching Exercises for Your Horse
You can bring the horse to stretch farther each time, and also use anything you want to direct him; I've found I can use a piece of hay, or even entice her to reach over to sniff flowers or follow my hand (I taught her how to point with her nose earlier).
Remember pressure and release, and keeping things short 'n sweet. End sessions on a good note, and know when to call it a day ;)