You should practice the question-and-answer session just as you practice the rest of your talk. When you rehearse your speech before friends and family, encourage them to ask questions—tough ones—at the end. Think up hard questions yourself. Get to know your subject so well that you anticipate possible questions, and get ready to answer them. Find your most argumentative colleagues and friends and give them a field day: Chances are they will throw trickier stuff your way than your audience will.
Anticipating questions in advance means you are unlikely to be completely stumped or taken by surprise by a question from the audience. You can't predict and control everything, but when you operate from a position of strength and full preparation, there is nothing wrong with simply answering, "I don't know, but I can find out." Remember that you're in charge of the speech and the answers. As Calvin Coolidge put it, "I have never been hurt by anything I didn't say."