Grasp your subject. The words will follow.
Cato the elder.

The real art of conversation is not only to say the right thing at the right place but to leave unsaid the wrong thing at the tempting moment. ~Dorothy Nevill

You can speak well if your tongue can deliver the message of your heart.
John Ford

"Words have incredible power.
They can make people's hearts soar,
or they can make people's hearts sore."

--Dr. Mardy Grothe

Be sincere; be brief; be seated.
~Franklin D. Roosevelt

Any man who makes a speech more than six times a year is bound to repeat himself, not because he has little to say, but because he wants applause and the old stuff gets it - William Feather

“A sage thing is timely silence, and better than any speech”

Plutarch

As it is the characteristic of great wits to say much in few words, so small wits seem to have the gift of speaking much and saying nothing.
Francois de La Rochefoucauld

All speech, written or spoken, is a dead language, until it finds a willing and prepared hearer
... Robert Louis Stevenson

Edward R. Murrow, and American Journalist early in the twentieth century said, “To be persuasive we must be believable; to be believable we must be credible; credible we must be truthful.”

Do you agree?

persuasive_believable