I use voice comands with all three of my boys.
"Whoa" (or "Hoa" if I need it in a hurry) for a halt.
"Steady" to take whatever gait I have them in down a notch...eg, canter to trot, trot to jog, jog to walk. ( I also draw the word out in a calm low voice if my horses energy is high)
To start from a halt to a walk I use "walk on", and any upward transition I use "get up" or "Hup".
These work well for me and my boys, but I think whatever phrase you choose, as long as you use it consistantly, your horse will learn the correct response.