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.
A case study examining asynchronous discussions in online graduate courses.
"Designing Online Discussions: Key Questions." Sheridan Center for Teaching and Learning.
Establishes a series of central questions for designing, responding to, and assessing online discussions.
"Generating and Facilitating Engaging and Effective Online Discussions."
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.