I don't lunge on a line very much, but I was taught that for upward transitions, use a rising tone to your voice, and for downward, a falling tone.
And, give a preperatory, "Annnd . . Trot!" just taking a deep breath before speaking a command can give the horse a warning that something is coming.
I hold the lunge line in exactly the same manner as holding a rein. so, if I am about to ask for a downward transistion , I will close my hand and firm up my forearm, just as I would for a preperatory half halt. For upward transitions, I kind of sink my body downward a bit , then lift it and open my chest to project more energy, and for down, sink again. IT's minute, but it helps me to keep myself "riding" my horse always, never just checked out and standing in the middle.