The art of rhetoric; or, Speak so that others listen to you

For every business person, their speech and their communication skills are the most effective sales tool. Regardless of whether you are into outside sales (face-to-face) or inside sales (over the phone), rhetorical skills form the core of success. Note that there are people with such good speech that everybody always wants to listen to them; then you have people that no one wants to listen to. How can you improve your speech and make your audience pay more attention?