To effectively use voice, gestures, text, and images.
For the Greeks, a good speaker was a good person. It was difficult for them to believe that eloquence could reside in an unworthy individual.
This idea seems naive to us today, especially after a parade of sliver-tongued, 20th century despots and scoundrels. In many cases today, we believe that too much skill in public speaking must be a sign of the speaker's deceptive ability and intent. How far we've come from that Greek ideal.
But, like the Greeks, we still find the ability to speak effectively, or write well, a prime source of entertainment. Anyone who would engage the public sphere on issues of civic concern would do well to consider the canon of delivery, i.e. the conventions of modern speaking and writing.