Historically speaking, [Demosthenes did that](https://en.wikipedia.org/wiki/Demosthenes#Speech_training), and it worked for him. "Plutarch also states that he had "an inarticulate and stammering pronunciation" that he overcame by speaking with pebbles in his mouth and by repeating verses when running or out of breath. He also practised speaking in front of a large mirror.\[9\]"