Model effective writing from your own students' work whenever possible. It's a good to do this even if some students are still having difficulty with a concept. For example, say you've finished teaching students how to write a summary, but the homework suggests that only eight students got it. You might decide to model two or three strong student samples in class.
Ask these individuals before class if they mind that you share their work (be sure to tell them that you are using their work as a positive model - it is never a good idea to put a student's problematic work on display for critique). Carefully plan out how you will facilitate this process. One approach would be to present a student's sample on an overhead and discuss what is working well in this piece with the class. Or you could ask the student to read their summary aloud. Consider other approaches as well, and decide which works best with your class and your teaching style.
Try to select work from various students throughout the semester. That way, students will see you're not basing judgments on one model for writing, but locating what's effective among various styles and approaches.