Read all of the materials in advance. Often times, a discussion will take place based on the topics covered in reading or homework assignments. Reading all of the material will give you a clear idea of what to expect during the discussion.
Take notes while reading. Any area of the reading that you feel is pertinent or important to the overall subject of the class should be noted. That way, you can reference those parts of the reading later on, during or in preparation for the discussion.
Write some basic questions and answers for things that you read which you may have found particularly interesting or did not understand fully. This will give you a go-to guide when participating in the discussion.
Speak up early. Do not wait around to be called on because some of your classmates might have the same topics to talk about as you and could beat you to a talking point.
Listen to your classmate's views. This will not only help you understand the discussion, but it will also ensure that you do not repeat the things that your classmates already mentioned.