Study the classics. I would say-well, you see it depends on what kind of voice-actor you want to be. There's two kinds-the more versatile kind, who can play all sorts of voices, and then there're those whose voices are really distinctive, like James Earl Jones. If that's what you've got, a really individual voice, then great, although I doubt even James was born like that-I'm sure he had to work at it. For myself, I'm in the favor of the more versatile kind of voice who can do all sorts of roles and transform. But to do that you have to have chops, and the best way to get them is through the classics, especially Shakespeare. It teaches you a lot about breathing, and idea and intention and rhythm. He's written the characters into the very poetry of the language, and if you try to get into and match his poetry it will take you out of your normal language patterns and that's the beginning of creating a character, not playing your mundane, everyday self. It's so verbally based, and in voice-acting, all you've got is your voice.