1. Finish your sentences.
2. Do not trail off at the end of sentences.
3. Learn to pronounce words correctly. My wife helps me with this!
4. Salt your speech with the Bible and great quotations.
5. Do not interrupt the other person.
6. Do not speak loudly if not necessary.
7. Make eye contact.
8. Develop and apt and ready vocabulary.
9. Try to avoid hackneyed expressions and worn out metaphors. See George Orwell, "Politics and the English Language."
10. Avoid stutter phrases and overused expressions. This is boring at best and annoying at worst. You lose credibility as a thoughtful speaker.
11. Read The Book of Proverbs, attending to what it says about the speech of a fool and that of a wise man.
12. Do not have gum in your mouth while speaking to someone else, even if you are not chewing. This is rude and can even make the other person nauseous. I know.
13. Do not speak so rapidly or slowly that this detracts from your demeanor and ability to communicate well.
14. Do not interrupt yourself. Even some well-known speakers do this constantly--Rush Limbaugh and Denis Prager, for example. This requires forethought and patience.
15. Protect your voice by hydrating enough and not speaking much when you have a sore throat. Yelling can also damage your voice.
16. Do not speak on a cell phone such that you interrupt or detract from unmediated discussions.
17. Do not repeat yourself if not necessary. Life is too short for that.
18. If you are funny, do not hide behind your humor.
19. If you seem to be more intelligent than the person to whom you are speaking, do not play on that.
20. If you seem less intelligent than the person to whom you are speaking, try to learn from the other person instead of being intimidated.
21. Try to speak as you would write (if you write well). See John McWhorter, Doing our Own Thing. He is a linguist.