I am surprised no one else has mentioned: know what you are talking about. The smartest people I have ever met speak well because they know the subject matter.

I am surprised no one else has mentioned: know what you are talking about. The smartest people I have ever met speak well because they know the subject matter. They are also comfortable admitting what they do not know. And as others have mentioned: read a great deal of well written books or plays and attend some writing classes.