I remember reading somewhere that students make up their minds about a course and an instructor within the first few minutes of the first class. Undergraduates rarely initiate conversations with teachers, so it takes real effort to make them comfortable talking with us formally or informally. They often find it hard to converse even with peers whom they don't know. It's a bit intimidating and depressing for students and instructor alike to walk the first day into a large, cathedral atmosphere by generating a relaxed conversational buzz from the start, greeting familiar students, introducing herself to new ones, and drawing students into chats with each other. Make the students realize that they count as individuals.
Anonymity is one of the largest barriers to active participation.
Students who feel the teacher doesn't know or care about them are less likely to participate than those whom the teacher recognizes as distinct persons with lives extending beyond the classroom. When students arrive on day one, I have them fill out a short questionnaire that asks for such information as their home town, major, career plans, and extracurricular interests. Chairs lined up in neat rows and all facing the lectern indicate that the course is lecture centered. If students do participate, they address questions and comments over the backs of their classmates' heads to the instructor. Try banishing the lectern to a corner and arranging the chairs in a circle so that students can see and talk with each other during discussions.