Happy Brainstorming has an introductory tutorial on vocal training.  Starting out with an example about the impact speech plays on social interaction, it continues exploring various techniques.  The article includes exercises as well as demonstrations of engaging speech.

You may want to be in a room by yourself when you try this.  If you start yelling “yeeeeeeeeeah,” in your office cubicle, your coworkers may look at you funny.  However, sitting in the privacy of my own home, I’ve found these examples extremely helpful.

Check out How to Improve Your Voice and Speech.