In business writing, you usually want your reader to do something. The question is therefore, how do I motivate my reader to act? The key lies in what would be of interest to the reader, what the reader's level of knowledge is, and what benefit the reader would get.
Rule 3: Motivation = Interest + knowledge + benefit
The first step in motivating the reader to read on and eventually do what you want is to arouse his or her interest. What is the angle that would make the reader go, "I need to know more"?
Next, ask yourself what the reader's level of knowledge is about this matter. If the level is high and the person therefore knows much, you won't give low-level information. If the person's knowledge level is low, you need to give enough information so that he or she understands you clearly and easily. If you were writing an article for a medical journal, you would assume a lot more knowledge on the reader's side than when writing for a lady's magazine.
Finally, tune into the reader's 'radio station': WIIFM – What's In It For Me? In other words, what benefit would the reader get from acting? Answer this by showing the reader how your message is relevant to his or her situation. How does the content of your message affect the reader, whether good or bad? What will happen if the reader acts, or does not act? Explain to the reader how your message is relevant to his or her situation.
To sum up: Motivate your reader to act by creating interest, giving the right level of knowledge, and showing a benefit.
PS: We've been discussing some introductory stuff up to now, and we'll go on doing so for a bit. Once the scene has been set properly, we'll go on to the 'bricks and mortar' of business writing: how to plan your document, get rid of excess baggage that clouds your message, write in the correct voice and tense, and dish up a nice and neat product to your reader.