Create a classroom system which involves every student in answering questions. This avoids several bright students from holding all the attention, and helps to engage students who wouldn't otherwise volunteer to answer a question. For example, this can be achieved by writing down the name of each student on a piece of paper and drawing a name from a box before each question is asked.
Make your questions cover a range of difficulties. This will provide an opportunity for stronger students to be challenged, but will also allow weaker students to cement their knowledge of the material.
Ask a mixture of questions which either require a straight, fact-based answer, or a thoughtful, opinion-based answer. This can help students engage with the material in different ways, and can be a useful method for provoking discussion.
Motivate students to explain the answer in their own words, and ask follow-up questions to inspire longer responses. These long responses are often useful, as they provide additional opportunities for the students to express their supporting knowledge and individual reasoning skills.
Challenge your students to think more critically about their answers. This can be achieved, for example, by playing the devil's advocate and critiquing the answers of your students against alternatives. This will also encourage your students to defend their reasoning when they reach a conclusion, and therefore have more self-confidence in their own answers.