I had an inquiry from someone about how to improve his sound. He had an issue that tons of people have: tongue tension and a sense that the sound is being placed or held back in the throat.
This post demonstrates one of the more dorky looking and effective ways to address this. This is what I call a cheat: it's a way to trick the body into doing something beneficial that you want to continue to do. You're not meant to always talk with your tongue stretched out; rather, you need to experience the open throat that this often causes and try to retain that when your tongue isn't stretched out.
One warning: I am constantly amazed at how many people think they are stretching their tongue way out of their mouths when they aren't! You need to use a mirror so that you can see if you are really doing this. Don't be surprised if your tongue tries desperately to go back into your mouth the moment you start to speak.
Another oddity is that people often give up completely on articulation the moment the tongue is out - keep trying to articulate clearly, even though it's nearly impossible!
FYI: The text I'm using to demonstrate is Shakespeare's Sonnet 29: "When in disgrace with fortune and men's eyes..."
Now start stretching those tongues, fools!