Students must feel comfortable talking to each other . This goes back to day one when you began to establish a relationship with your class. Knowing your students and having students become familiar with each other will greatly increase the chances of discussion. Some teachers start out at the beginning of class having their students develop a criteria for good discussions. This then becomes the standard for class discussions.
Students can also be responsible for developing discussion questions.
As strange as it may seem, some students need to be taught how to be part of a group discussion. Many have developed the habit of filtering all of their comments and questions through the teacher instead of talking to other students. Volleying questions and answers back and forth with the instructor is not a discussion - even if a good number of people are participating. The instructor may be part of the discussion as any other member of the group without dominating. Respond to questions and comments in a way that indicates you have heard and understood what was said.