Establish a tone in the beginning of a class that encourages discussion and communication. For example, on the first day of a college class, you might ask students to explain why they have taken the course. In a class in high school, you might ask students to explain their biggest hope and fear in regards to the class. This allows each student to speak out loud and gets students used to talking in class.
Explain to students that you expect a certain level of communication in the class. You can go as far as to require a certain number of comments from each student per week, or you can simply state that you encourage classroom participation. Either way, make sure that students realize that their communication is welcome and expected.
Present the curriculum in a way that encourages discussion. For example, don't spend the whole class period explaining an idea. You might start by asking students about their thoughts on a particular topic, after which you can offer more information that caters to their various questions or comments.
Think of alternative ways to get shy students to communicate. For example, you might want to have a journal component in class, in which students can inscribe their reactions to or thoughts about the subject matter.
Avoid yes or no questions, because these do not offer the opportunity for a lot of discussion. Instead, ask questions that encourage students to explain what they mean. For example, do not ask students if they think "The Sound and the Fury" is interesting. Instead, ask students what parts of "The Sound and the Fury" capture their attention and why this is the case.