Look at the diagram below. After the appeal of each of the developmental elements, there is a column with the aim of the development material in each case. By aim I do not mean what you hope will be the outcome of the sermon. I mean rather your own aim for the material you are preparing for your sermon. Each form of development aims to present the sermon ideas in a certain way and with a certain impact. Each makes a certain contribution toward communicating the ideas and persuading the hearer to embrace them.
Your aim with explanation: Present the ideas as clear
Explanation bridges the gap between the historical world in which the text was written and the world of the contemporary audience. It ties your sermon truth firmly to the text and confirms to the hearer that you are preaching from biblical authority and not just opinions. To do this you must go back and explain that biblical world so that your audience better understands it. You explain how what God said then is what He is saying to us now in the timeless universal principles you have presented in your outline division statements.
You simply want to plan your explanatory material in such a way as to interpret the sermon ideas as clearly as possible. Clear means "easily seen or comprehended, free from obscurity, easily intelligible." Not only do you want the historical record of the message to be clearly understood; you want the timeless truths of the text to be clear. Unless the ideas are clear, other kinds of development are of little use.
The work development
Your aim with illustration: Present the ideas as vivid
A clear understanding of any idea requires that your hearer be able to visualize it. The most common kinds of illustration are examples, like testimonials, of people applying or denying the point, and analogies, using parallel images from outside the religious dimension. Like opening a window to let light into a dark room, illustrations shed light on your sermon ideas so that the hearer can see what you mean. Illustration demands imagery; no imagery means no illustration. Test every illustration for specific, concrete imagery.
Your best way to appeal to the imagination of your audience is to use vivid language, to draw pictures.
Your material is vivid when it brings strikingly real or lifelike images to the mind of the hearer. These images give life to the ideas of your text. They leave an impact on the imagination so that the concepts will not be easily forgotten.
Your aim with argumentation: Present the ideas as plausible
Most preachers speak to an "in house" crowd, to those who are already convinced. Our hearers, however, may not be as convinced as they seem. They live in a secular world where conflicting ideas constantly challenge their Christian faith. They need evidence to support their convictions. They need to know how common criticisms and accusations can be answered. Since a sermon is not a debate or a philosophy seminar, you will want to avoid complex and abstract arguments. A simple, direct, "common sense" approach will be much more convincing.
It may seem at first that plausibility is a rather weak aim for this part of your development. You may rather want to aim for undeniable proof. Though that would be a great thing to achieve, I doubt you will ever produce material that cannot be challenged at all. Plausible means "seemingly true, acceptable." If your argumentation achieves that level of effectiveness, you will have overcome logical barriers and opened the way to faith. That is enough.
Your aim with application: Present the ideas as practical
Application is more than just attacking the congregation with the sermon truth. Application presents the implications of biblical truth for the contemporary audience. It is a call for action, for putting the principles of Scripture to work in our lives. It deals with attitudes, behavior, speech, lifestyle, and personal identity. It appeals to conscience, to values, to conviction, to commitment to Christ. (See Howard Hendricks and William Hendricks, Living by the Book, Chicago: Moody, 1991, p. 304ff for nine rather specific questions to ask for application.)
The most common failure of application is that it is too general, too religious, and too vague. The general applications of most preachers do not really connect with the hearer's sense of what real life is about. Avoid sweeping criticisms. Aim rather for your application to be practical. Deal with real life. Give concrete suggestions as to appropriate changes in response to the ideas of the text. Show how your hearer can express his faith and experience the grace of God.