Let me emphasize a point I made earlier: Anyone—regardless of his or her vocabulary—can be an outstanding speaker. You don't have to know unusual or complicated words to use power language. The important thing is to use language that is comfortable for you and to use it in a creative, colorful way.
If you do use complicated words, make sure they're completely familiar to you. The president of a football team forgot this rule when he introduced a recently acquired player: "His influence on the state's economy will be inconceivable," he crowed. Then he thought a bit. "I mean incontrovertible. No...inconsequential. Well, here he is." Misusing a word in an attempt to appear learned has led many speakers into a Malapropism: "My wife tells me I'm an invertebrate smoker."
It's not only complicated words that get us in trouble. Politicians are well known for using the wrong word in the wrong place. Mayor Daley (again) once said, "The police are not here to create disorder. They're here to preserve disorder." Many people questioned the intelligence of President George W. Bush when he said things such as, "I know how hard it is to put food on your family," "Will the highways on the Internet become more few?" "I have made good judgments in the past. I have made good judgments in the future," and "The future will be better tomorrow." And then there is the champion of the misused word, former Vice President Dan Quayle, who once said, "Republicans understand the importance of bondage between parent and child."
Sometimes a slip of the tongue is good for a laugh. Kenneth Keating was once invited to give a speech with this charming invitation: "I hope you can come, Senator, because we would all like to hear the dope from Washington." Senator Keating turned that into a classic story and used it repeatedly in future speeches. Will Rogers was one gifted speaker who used the wrong word on purpose very effectively: "In some states they no longer hang murderers—they kill them by elocution." In both cases, the speaker used a word that surprised, and that came at the end of a sentence—a perfect combination for memorable sentences.