If you have given this presentation several times, you have probably heard most of the related questions. This allows you to work out the perfect answers, with memorable phrases and without repeating what you already said in the speech. You might want to illustrate a point you barely touched on with an anecdote or example. Rehearse these answers as well as the formal presentation. Other hints include:
Ask colleagues and authorities in the field what questions they commonly get on the same subject.
Scan new books for points you have not addressed in your speech.
Read trade journals more closely.
Think about what else you might have covered if your allotted time had been twice as much.
Bring documentation you can refer to in order to support your arguments, like the detailed results of a survey you may have just alluded to.
Leave a couple of obvious questions unanswered to be sure you get some response, which will stimulate others to think of questions. If there are no questions, you can always say, “One of the most common things people ask me about …,” which will give them more time to think about what they might want to ask.
If the question asked is really irrelevant, politely say that that subject is beyond the bounds of what people came to the lecture to talk about and point to the next person with her hand up.