Vonderwell, Selma, Xin Liang, and Kay Alderman. "Asynchronous Discussions and Assessment in Online Learning." Journal of Research on Technology in Education 39.3 (2007): 309-28.
http://files.eric.ed.gov/fulltext/EJ768879.pdf
A case study examining asynchronous discussions in online graduate courses.

"Designing Online Discussions: Key Questions." Sheridan Center for Teaching and Learning.
https://www.brown.edu/sheridan/teaching-learning-resources/teaching-reso...
Establishes a series of central questions for designing, responding to, and assessing online discussions.

"Generating and Facilitating Engaging and Effective Online Discussions."
http://tep.uoregon.edu/technology/blackboard/docs/discussionboard.pdf
Prepared by the creators of the Blackboard, a Learning Management System (LMS), but some of the practices described can be generalized to other environments as well.