People commonly write for any of six reasons: (1) to express themselves, (2) to inform others, (3) to explain things, (4) to criticize, (5) to argue a point, and (6) to entertain. Nos. 2–5 all have a necessary place in persuasion, and no. 6 (at a subdued level) is a definite plus.
To persuade, you must prove things to your readers’ satisfaction by emphasizing evidence, logic, and reason. You criticize contrary positions, preferably without caricature. To the extent possible, you also appeal, subtly, to your readers’ feelings, values, and self-interest.
When writing persuasively, you undertake a kind of journey—a walk with the reader from here to there. Every word, every sentence, and every paragraph should be a step toward the destination, with deftly placed signposts along the way. The quality of your finished writing turns on your seriousness and intelligence, your ability to focus and organize material, your knowledge of English usage, and your patience to stay with the project until you’re justifiably happy with it. The journey from start to finish takes time, but if you begin with a purpose in mind, and you pay attention to what you’re doing along the way, achieving your goal can be enormously satisfying and rewarding.
Naturally, that’s true for your clients as well as for you.