Nya says that "gait is all about head set." If your gaited horse raises her head too high, she will start to pace; too low or too tucked, she will trot.
We could see this in action with all our horses as we experimented with this. Depending on how high or low I asked Piper to hold her head, I had her running the gamut from pace to stepping pace to flat walking to run walking to foxtrotting all the way to trotting--and what a bouncy part of the ride that was!
It was not breed specific. I had my Tennessee Walker and my friends had a Mountain horse and a Foxtrotter, respectively. I believe all of us achieved the same results--proper head-set resulted in a nice, smooth ride.
How do you best use this information? Determine where your horse holds his head when he is giving you the smoothest gait. Each horse will be slightly different. Practice having your horse maintain that headset.
If the ride gets rough, reset your horse back to the ideal. If his head comes up, gently see-saw the reins to encourage him to bring it down slightly. Add pressure with your legs as needed so that he understands you are not asking him to slow down, just to bring his head down.
If your horse gets too trotty, jiggle the reins slightly to encourage him to raise his head slightly. I recommend you teach your horse to raise and lower his head from the ground, so that he has an idea of what you want before you get on.