The audience will process your speech more easily if you provide an appropriate organizational pattern: an introduction, body, and conclusion, each with clear transitions.
When developing an organization for your speech, consider first how well it suits your purpose, audience, and constraints such as the occasion, time limit, and venue. For example, if your purpose is to inform, your goal will be to increase the audience’s understanding and awareness of a topic, but how you go about it will depend on your audience. If they’re not familiar with the topic, you’ll need to provide background information. But if your audience is well-versed on your topic, you wouldn’t want to bore them by repeating what they already know.

Introduction

Your introduction should grab the audience’s attention. Use vivid language, imagery, and interesting facts to keep your listeners interested, but exercise caution when using humor since it can backfire unless you really understand your audience. For your opening, you could cite a startling statistic, pose an open-ended question, tell a story, use an apt quotation, or relate your topic to the audience to establish common ground. Next, clearly state the central idea or thesis of the speech.
You may need to establish your credibility and gain the audience’s confidence. Why should they believe what you’re going to say? If you have firsthand experience or have conducted research on the issue, let your audience know that. In addition, make your topic relevant by discussing its significance. What will your audience gain from understanding the topic? Does your topic have practical application to their lives? Will you provide them with the information and/or motivation they need to accomplish a goal or take action?
As a transition, preview points you’ll address in the body of the speech. This preview signals that the end of the introduction is near and the body of the speech is about to begin.

Body

The body of your speech is where you’ll present your main points. Each main point should be supported by information or research. You can use examples, narratives, testimony, facts, or statistics as supporting material. A number of common and useful patterns can help you organize your main points.  Remember to select a pattern that suits your purpose, your audience’s needs, and your specific speaking situation, including any time constraints you may be under.
Chronological (sequential). You can order the main points of your speech by time. For example, you could describe an event in order from beginning to end (the history of Texas A&M University) or explain the sequential steps of a process (the development of the influenza virus vaccine).
Spatial. When you want to describe the structure or physical makeup of something, you can use space as your organizing principle—from top to bottom, left to right, east to west, etc. For example, you could discuss the major wine growing regions in France by beginning with the regions in the north and working toward the south.
Causal (cause/effect or consequences). The main points in this pattern show a cause-effect relationship (e.g., the causes and the effects of the oil spill in the Gulf of Mexico). Decide if you want to focus on causes or effects. Let’s say your topic is the use of pesticides in farming. You could focus on only effects by explaining the effects of pesticide use on soil.
Alternatively, you could focus on only causes by explaining why farmers use pesticides. In many cases, this makes sense because the effects are obvious; for example, in discussing a deadly airplane crash, you might limit your presentation to exploring the multiple causes of the accident.
Another possibility is to discuss both causes and effects. Usually, you begin with causes and end with effects. For example, farmers use pesticides because they need to increase crop yield on less land or because chemical companies pressure farmers to use them. The effect of such use is that the pesticide residues pollute runoff and then streams, rivers, and lakes.
Problem/solution. This pattern will usually have two main points—one showing that there is a problem and addressing its seriousness, and the second one providing a feasible solution to the problem (e.g., the dangers of food additives and how to avoid them, or car accidents and how to decrease them).
Comparison (similarities/differences). Use this pattern when you want to highlight similarities and/or differences between two events, objects, or situations (e.g., comparing two presidential candidates). You have two options for organization: whole-to-whole and part-to-part. In both cases, first decide on the points of comparison. If you are comparing two candidates running for political office, you might select as points of comparison their stances on taxes, health care, and the state of the military.
In a part-to-part organization of that information, you’d have three main points: (1) Stance on taxes for candidate A and B; (2) stance on health care for candidate A and B; and (3) stance on the state of the military for candidate A and B. In a whole-to-whole organization, your first main point would be candidate A’s stances on all three issues, and your second, candidate B’s positions on the same issues.
Categorical. This pattern allows you to divide your main topic into categories. You would discuss each of the categories or explain how the topic fits into the various categories (e.g., different breeds of dogs or types of movies).
Analytical (Topical). Divide something into logical parts and then discuss how the parts work together for an effect. For example, you might analyze a song by breaking it into lyrics, melody, and harmony. Or you might analyze a basketball game by breaking it into offense and defense.
Monroe’s Motivated Sequence. This organizational pattern, developed by Alan H. Monroe in the 1930s, is designed to inspire people to take action. It consists of five steps:

1. Attention: Gain the attention of your audience by telling a story, using a startling fact/statistic, posing a question/rhetorical situation, using visuals, etc.
2. Need: Show that there is a need for change and that the situation at hand is dire by using statistics, examples, testimony, etc.
3. Satisfaction: Offer a detailed solution to the problem you just presented.
4. Visualization: Help the audience visualize the benefits of your proposed solution.
5. Action: Tell the audience what action they can take to solve the problem.

It’ s often used in television commercials and persuasive speeches, especially for organizing policy claims, such as persuading a community to install red light cameras to improve safety at an intersection.

Conclusion

In your conclusion, let the audience know that the speech is ending (for example, say: “In conclusion,” or “To sum up”). Review the main points and central idea of the speech (your thesis). End with a note of finality (by using a powerful quotation or coming up with a dramatic statement of your own) and challenge the audience to respond. Keep in mind that your conclusion shouldn’t be too long—ideally no more than five to ten percent of the speech.

Transitions

To hold each section of the speech (introduction, body, and conclusion) together, you’ll need to craft some transitions. Transitions are particularly important in a speech because listening is harder than reading. Good transitions guide the audience and keep their attention focused. For those whose minds wander, a good transition can pull them back into your sphere of influence.
Transitions should summarize the previous section or main point and then introduce the next one. They usually are no more than one or two sentences, and they use words such as first, second, third or in conclusion, on the one hand . . . on the other hand, next, now, finally.
Ex. Now that we have seen how pesticides are poisoning our ground water, let’s consider why farmers continue to use them.
Ex. I have explained that Councilman Connors believes we should not raise taxes; next, I will explain why Councilwoman Hurston is advocating a higher tax to support education.
Ex. Greeks are proud of their classical past. Let’s take trip now to modern Athens to see how they are preserving their antiquities.
 
Ideally, you should provide a transition after the introduction, between each main point, and before the conclusion. Typically, after each main point, speakers use an internal summary—a brief restatement of the point.
Repetition of key points is a good way to hold the speech together. Remind the audience now and then of the thesis of your key points.
Ex. Remember, former Aggies are intensely proud of the university’s history and traditions. So when an event like the collapse of Bonfire occurred, it was a tragedy on many levels, personal and cultural. [In a speech about the history of Texas A&M in which the thesis is that Aggie culture is influenced by former Aggie’s beliefs about tradition.]
 
Another way to think about transitions is to consider their function. Some transitions just link, or show the relationship between the ideas. Others forecast what is to come; and still others summarize what you said previously. Each is effective, and when used properly, can have a profound effect on the understandability and strength of your speech.

References

Lucas, S.E. (2008). The Art of Public Speaking (10th ed.). McGraw-Hill: New York.