Here is an informative article on how to motivate students to write. However, I've found the best way a teacher can engage students in writing is -- is to be a writer himself. When I show my students some of my work, when I then ask them to create their own and we will work on it together to polish their work -- I find the most motivated students.
     Teachers who are writers also far better understand exactly what they are teaching. How often I've heard students come to me from their regular classrooms, giving me the classic definition of "metaphor." Their own teachers did their best, I'm sure, when they armed their students with a definition to explain this wonderful writing tool. But did they tell their students how metaphors liven up writing, how they allows the reader to see an image in the environment they themselves live in, how the ordinary sentence become extraordinary in the hands of a metaphor? A writer would tell students this -- and more.