I summarize below some of the readings on the topic. Emphasize participation over content early in your use of the discussion forums. This minimizes the student's perception of risk in using the forum. Create activities that encourage students to use the discussion forums. Have students debate a topic on-line.
Students can post questions, and even reply to other student questions.
Distinguish between informal and formal discussions. Make the conference integral to your course. Make sure you pick up on the on-line discussions and bring issues raised to the physical classroom. This shows the student that the discussions are important to you. In the other direction, suggest that unresolved issues continue on the conferences after class.