I like Doug's answer - and use an extension of that - I don't answer with a straight answer, I say, "What do you do when you aren't here - any hobbies? OR What do you do for a living?" - this gives me a chance to get to know them. When they respond, I think about their job and say, "How did you learn that"? They may say, "Trained by others, read a book on it, a family tradition"...anyhow, I just say - Magic is much like that - anyway you can think to learn, you can - but the stuff I am showing you is well gaurded.
This acknowledges that magic is exposed, but the best stuff (hopefully, YOUR stuff) is not "public domain".
NADZRI's response, I have a hard time NOT agreeing with, however, I think audiences may be insulted sometimes by having you sidestep a sincere question. I have gained MORE respect saying, "I study a book at Chapters...but the difference between me and most, I have read that book again and again - read between the lines...so much the pages fell out and I had to buy the book again" - rather than - "Where did you learn that?" and the reply a joke like "I went to school with Harry Potter". It will get a laugh, but that doesn't always convey success at connecting and dismissing a serious question.
The former, again, really answers their question - the latter just makes a joke, and although a great way to entertain and avoid...I think it has poor effect.
Anyhow - once again - I turned a short answer long - sorry.