How do you teach writing in a scalable manner to third- and fourth-graders?

This is a bleg, so please leave your sagacious answers in the comments.  Kelly Smith, of Prenda, writes me:

To summarize, I want kids to love writing, to see themselves as writers, and to improve their skill. I’d love to know about anyone who has done that systematically.

Are you able to help?  Rest assured that your answers will be put to good use.