U.S. President John F. Kennedy gave this speech in Berlin in 1963. He was assassinated later that year. Although a great deal of preparation went into the speech, Kennedy, clearly fed by the energy of the crowd, gave a far stronger speech than had been intended politically. The speech uses rhetorical repetition very effectively, not only with the famous “Ich bin ein Berliner” line, with which Kennedy closes the speech, but also by repeating “Let them come to Berlin” many times in the opening.
Kennedy uses the divided city of Berlin as his core conceptual metaphor. This speech was given two years after the wall dividing the city had been constructed. The Berlin wall was a clear metaphor for the freedom of the West, compared to the lack of freedom in the East, in Kennedy’s mind. He uses this clear metaphor to drive the message of the entire speech. It is so memorable because it is so easy to picture. The existence of the wall makes the point for Kennedy. If you can find something as strongly demonstrative of your point, do not be afraid to use it again, and again, as Kennedy did.
See here for full speech.