At the end of most presentations, you are pretty obligated to ask, “Any questions?” Are you actually ready for those questions though? Matt Schmid writes a short article with advice. First of all, thank the person for his or her question, no matter how hard and utterly damning to your career it is. Second, clarify with the person that you understand the question, even if all you are secretly doing is trying to buy time while you contrive an answer. Third, consider one of three options in giving your answer: admit you do not know and ask for help from the audience, admit that someone else you know would be better qualified to answer and get back to the questioner with an answer later, or say you need to gather more information and ask to meet privately later in order to provide the best answer.
You can read the full article here: http://www.targettraining.eu/handling-difficult-questions/