Great question and something that many instructors struggle with. Here are my suggestions:

Environment: First it is important that ensure that a good environment has been established so that everyone feels comfortable and safe. I begin my course by quickly talking about the experiential learning cycle and expressing how we are trying to develop a community of inquiry where we are learning from one another. I also tell them that a participation grade is given to encourage everyone to engage so that we can help develop this community and create greater emotional connection with the content for deeper learning and better recall. It is also important that everyone knows that students are not to degrade anyone's response and that you will not be get angry if someone provides the wrong answer or asks a question that doesn't necessarily make sense at first.

Instructional Presence: I am also very aware of the way that I am presenting myself so as to be as approachable as possible and to help create a cooperative/engaging environment. I use active listening, good verbal and non-verbal communication, and present a pleasant/genuine smile in order to better connect with everyone.

Personalization: I try to use specific examples and questions that involve local issues, current events, and aspects associated with my students' majors or interests. I also work very hard to address students by name (this has a psychological affect that also encourages interaction). Relevancy is also extremely important and should be explicitly expressed to encourage more student interaction.

Question Types: I also warm up the students by asking them easy questions to begin with, and also do simple polling (raise your hand if you think this or that). As I ask different questions I make sure to ask different types of questions in different ways: multiple choice, true false, short answer, etc., ensuring to give them enough time to think before answering. I also sometime ask a student directly or sometime open up to others. I usually do ask if anyone has any questions, but if no one does, I often ask them to come up with questions that a student might have.

I have found that by combining these things it greatly improves the overall class and helps students interact and ask questions.