Ensure you leave enough time during the class for questions, preferably throughout the course and not just at the end. If the class is rushed, you are less likely to encourage students to ask questions and they won't want to risk prolonging the lecture and annoying other students.
Let your students know you won't make them feel foolish for asking questions, and that you really want them to thoroughly understand the material. Create a supportive environment, making it clear that you won't tolerate teasing or laughter from anyone in the class when a student asks a question.
Answer all questions in a friendly manner, even if you have answered the same question already. Give new examples to illustrate your answer and find different ways to help the students understand the material.
Ask all of your students to write down at least one question part-way through the class, or at the end of class, and read the questions out loud without identifying who asked them. This shows the entire class that everyone has questions, and helps build the confidence to ask out loud.
Keep track of the questions you get from students who come to you outside of class hours, and bring these questions to the class without identifying the student who asked. Encourage students to ask these questions in class so everyone can benefit from the answers.
Play games like '20 Questions' with younger students, to show them how asking simple questions can solve problems and riddles. Encourage them to ask who, when, where, why and how.