Maybe you could give your horse rhythm with your voice, like setting the tempo with your voice or with a whistling or something like that.
I do it when I ride my horse and it really works, I've never tried while longing him yet though
I have a horse that always takes off going right on the line which 1. Isn't safe and 2. Not fun. I would love to have any tips myself. One thing I do try to do is always make sure their inside ear is focused on my voice, and if he is being good I try to have a soft positive tone so he knows that's what I want.
I do a long 'aaaaaand stand' (sometimes with a whhhhhooaaaaa in front) and each time I ask for a downward transition I turn my body from facing out at the horse to sideways on to the horse, so I go from big person, to small person.
Keeping this in mind, you need to show the horse that what you are asking is not going to be too hard for her to handle at the point in training where she is at. If you ask her to do too much, she will begin to regret following your cues in the first place.