I am a PhD student and I'll be giving my first talks at an international conference soon. 

In the past I have had tough questions from the audience that I did not know the answer to, or did, but it didn't come to mind until after the talk. 

I have also had occassions where I hadn't heard the questioner twice, after repeating, and I couldn't ask for a third time (I am a bit deaf). As a result, I responded with an answer to a question that might have been asked, which was sort of a gamble. 

I am curious, how do people best deal with these situations during talks? are there any 'formulas' of how to deal with this? what methods do people use to relax during question time?