Use repetition as a mnemonic device. Early speakers and singers used repetition to aid their memories while relating key points or stories. Some modern speakers, and most recording artists, use repetition for similar purposes. Observe how repetition is used as a memory aid in ancient works such as "The Illiad," "The Odyssey" and "Beowulf."
Use repetition to emphasize points. This is one of the more common usages of repetition in modern rhetoric as it creates coherence between paragraphs when used for transitions. Emphasis is also the most common use of repetition in educational rhetoric to drive home a point. Repetition used for emphasis is best seen in speeches such as Martin Luther King Jr.'s "I Have a Dream" speech, or in documents such as the Declaration of Independence, written primarily by Thomas Jefferson.
Create rhythm in a speech, paper, poem or song with repetition. Similar phrases appearing in succession will either soothe the listener or build to a climax. Rhythm is more common in oral rhetoric, whether speeches or songs, because the rhythm can be controlled by the speaker's inflection and speed. Repetition for rhythm is best seen in most music (the chorus) and political speeches such as President Kennedy's inaugural address.
Use repetition sparingly for greatest effect. It requires a balance, because too much can be jarring for the audience, so erring on the side of too little is best. Synonyms can be used to create a more subtle form of repetition as well. Finally, be sure that the rhythm is appropriate to the audience and subject or that the points of emphasis deserve to be repeated.