Kern River Estate said:

Simple and easy method:

Give the slow or stop command, if he doesnt do it, step torwards the front to make it more apparent what you want, if that doesn't work, draw your lunge line in to make the circle smaller till they stop and reward the behavior.

Rinse and repeat as needed

Belle said:

I normally have my guys start to trot, then watch when they start to slow themselves. When I see them begin to drop into walk, I do a slow 'Waaaaaaaaalk' until they do it themselves. It makes it think it's their idea then and they usually get it eventually. Mine all go by voice while on the lunge so seems to be working lol