Even though our students learn basic persuasive writing skills long before they come to school ("I'll be really quiet if you buy me that toy"), they don't come to us knowing how to write persuasively. Writing is different than speaking. To persuade through writing, students need to analyze how they successfully convince others through speaking, then combine those skills with solid writing instruction.
What we've learned is that there must be a strong foundation of other writing skills in place before asking students to write something persuasive. Without the foundation, the persuasive writing your students will do will be flat and uninteresting.