Get in the habit of asking people who have heard you whether your purpose was clear. Before you give your next talk or chair a meeting, write a sentence that you feel describes your purpose. After you deliver your presentation, ask the participants what your purpose was. They should be able to tell you—easily. See how closely what they say matches what you wrote. If it doesn't, you need to work harder on making the content support and evoke the purpose you have in mind. Without feedback, you can only assume your purpose was clear—you'll never know for sure.
Focused, committed, invigorated—isn't that the kind of speaker you enjoy hearing? It is the kind you can become, and the first step is that firm grasp of purpose. Determining your purpose really is an easy step, and it makes everything that follows, including organization and selecting good supporting material, much easier. When you communicate a strong purpose, people see you as a leader with vision, which can't help but add power to your presentations.
One of the greatest benefits of having a clear purpose is that it helps you cut down on your preparation time. Recently, I was working with an endocrinologist who was working on a speech he was going to present to other doctors. He spent a lot of preparation time gathering technical information he thought he would use. But when he realized that his real purpose was to persuade these doctors to treat diabetes more aggressively, he also realized he didn't need as much technical information as he originally thought. Had he stated his purpose in the beginning, he would have saved himself hours of prep time.