I have no idea if this is a universally accepted exercise, but it's something I started doing not too long ago.

I'll find characters in movies who have really memorable, intriguing voices that I like and I'll record a few of their lines and throw them on an mp3 player. It helps to study what makes their voices so powerful sonically and it's a different way to improve your own if you recite their lines and sort of imitate their voice. Right now I'm a big fan of Mathew Goode in "The Lookout". He plays a master con artist and his voice is a powerful tool he uses to persuade people. Every word he says has a certain presence to it. A skill like that is paramount in seduction.

I used to hate my voice honestly (which is why I was so apt to improve it), but now I can project a cool, laid-back, airy sounding (if that makes sense) voice that can reek of confidence if used properly.

I personally believe vocal tonality is very important. Remember, "It's not what you say, it's how you say it."

And if you master how you say it... Well... The possibilities are endless.

EDIT: For anyone interested, I uploaded a few of the tracks I have from "The Lookout" HERE.