Answering questions is an excellent time to reinforce your key points. If your response can be enhanced by referring to a slide or overhead from your speech, bring the visual back up. Respect the questioner. If your listener asks a good question, say so. If he or she has a different viewpoint from yours, disagree without becoming disagreeable.
Diplomacy is essential here.
Feel free to postpone an answer. Sometimes you're just not going to know the answer to a question. In such a case, offer to research it and follow up with the audience member at a later date. Then do it. Do not under any circumstances try to bluff.