Advice: slow down when you are talking, don't try to play catch up between your mouth and your thoughts. You will stutter and "umm" "hmmm" less automatically. Also because of this it is likely that your mannerisms will become more coordinated and your voice will be a bit lower, which will add to the "eloquent" image.

Exercise: Pick a topic and talk about it for 5 minutes in front of a recording camera on your computer (on a Mac QuickTime can do this). Then watch how you look and sound, notice the things you don't like, adjust them, and iterate.