Extemporary Preaching

A man who is well instructed and who has a great facility of expressing himself; a man who has meditated deeply in all their bearings upon the principles of the subject which he is to treat; who has conceived that subject in his intelect, and arranged his arguments in the clearest manner, who has prepared a certain number of striking figures and of touching sentiments which may render it sensible and bring it home to his hearers; who knows perfectly all that he ought to say, and the precise way in which to say it, so that notihing remains at the moment of delivery but to find words to express himself--such is the extempore speaker.
(From Fenelon's Dialogues on Eloquence, quoted in the Concise Encyclopedia of Preaching.)

Or as my homiletics professor at Baylor would put it, "Do your studies and then preach from the overflow."