When someone asks me to show them a trick, my usual response indicates that I'll do something later for a larger group. I get a lot of people who ask at parties and such, and the easiest way is to perform once for anyone who cares to watch. Because of this, I go to one (or more) of about 10 card tricks that I've practiced and perfected with a borrowed deck. Try not to become a magic monkey that performs whenever someone asks. Magic is special, so you have to treat it that way. I make my magic a performance.