I, too, would be concerned about hitting the horse in the mouth as she went over the jump. What I use (and see at many shows) is a chain UNDER the chin and attached to the upper halter ring just as you would do for showmanship. I find this gives a bit more control but cannot hurt the mouth. You do need to be careful, however, as too much pressure under the chin can cause a horse to rear.
What we do with our gelding that is pretty excitable, is trot him around the course - not over the jumps but around the outside - until he is calmly trotting. His "reward" for trotting calmly is letting him jump! We keep repeating this as needed. I would suggest that you work on something like that to tone her down a bit and reward her if she goes calmly over the jumps.
We train some of ours for both hunter and jumper so we use slightly different cues depending on the class. For a hunter course, where I want a nice steady trot, I will remind them to "trot" between the jumps, saying the word on entering the ring and on landing each jump. For jumper, I will move faster (as fast as my old body can go) and cluck to indicate they are to go faster. I am sure there are other training methods out there, but that works pretty well for me.
