From the Teaching Center at Washington University in St. Louis:
Active student participation does not happen naturally in university courses; it must be carefully planned and encouraged. Set aside time throughout the semester to assess student participation in your course and to develop strategies for improvement; administer midterm student evaluations to help you with this process. Consider asking a colleague to observe your class; often, outside observers can discern patterns that hinder participation but that may not be apparent to participants. Take notes during and after a semester so that you have a record of what went well and what you would like to change the next time you teach the course in order to increase student participation.