Obviously you may need to be a little larger than life when you speaker in public, but avoid the over-theatrical and melodramatic. On radio and television, on the other hand, you'd be well advised to be a little‘smaller than life' ,while not becoming so subdued as to seem somnolent.
Public speakers are like conjurors: to get their effects right they need to practice, practise,practise. But, unlike conjurors, the best practise for public speakers is that done before an audience. Without any doubt, the more often you speak the better you'll get.