Every year the time comes when I need to teach the students how to write persuasively. We look at different examples of persuasive writing, and discuss the different arguments that are made in examples.
When it comes time for students to write,it always helps if the students know that if they are successful at persuading me to their opinion they will get something they want.
I often start by giving topics like, write to convince me that we should go to P.E. today, or write to convince me to do an art lesson tomorrow. Often times I already have PE or Art planned, but it motivates the students. If the whole class is writing on the same topic, I will often pull a couple of papers from the pile of paper to review with the class. I don't show the name of the students whose paper I review with the class. After reviewing a couple of paper I'll often praise their effort and tell them they have earned what ever they were trying to earn. Having a set goal for the class to achieve is a great way to motivate students.