Instead of using rubrics, I prefer negotiating writing criteria with students. Criteria are organic because they emerge from what students already know and what I'm hoping they'll practice and learn; they are positive and change with each piece students write. Eventually, students learn to create their own criteria for free-choice writing, setting specific standards for themselves. Students negotiate content and some style and organization criteria after studying one or two mentor texts. I might add a style criteria and suggest a writing convention criteria based on my observations of students' writing needs. Because you negotiate criteria before the planning stage, you know early on the mini-lessons you'll need to teach as the unit progresses. See Teaching Nonfiction Writing: A Practical Guide
(Scholastic, 2010) and Teaching Middle School Writers: What Every English Teacher Needs to Know
(Heinemann, 2010), for a detailed discussion of criteria.