Public speaking became much easier to me once I recognized that all good speeches follow a concrete formula.
It's kind of like writing. You wouldn't pick up a pen and start scribbling a lengthy essay without considering its structure.
Similarly, effective public speakers follow a pattern — not necessarily the same formula, but a formula. For example, Bill Clinton likes to...
1) Begin with a personal, visual anecdote about a specific person or small group. (e.g. A family walking miles to collect water.)
2) Relate the small example to broader theme. (e.g. Poverty is a big problem.)
3) Weaving that broader concept into the theme of the speech.
Another thing to remember is that while speeches share a structure with writing, they are not written articles. The biggest difference, I think, is that people are not capable of processing as much information.
While repeating yourself in a written piece is often bad form, most public speakers repeat key phrases to keep the audience focused. Listening is usually harder than sitting down to read.