There was a rather good article on the Wall Street Journal site, How to Train Your Voice to Be More Charismatic. It also includes a short podcast. It is behind a paywall but you can access it for free through Google. Here is a link to a google search, pick the first WSJ link.

Edit: I am including a link to a PDF from Toastmasters, Your Speaking Voice: Tips for Adding Strength and Authority to Your Voice. This should provide more practical exercises to enhance the WSJ story.