There are one of two roles you will be playing when attending a house party or get-together, the first is as a hired entertainer and the second is as an invited guest. I'm sure you already know what to do when someone hires you for a gig but, what do you do when you're just a guest at a party.
You will probably be called upon or coerced into performing on your "night off" at some point during the event. There are those performers who have adopted the mindset that they will refuse to perform under these circumstances because they are not being paid. This is a foolish and unfortunate position to place yourself in and a horrible business and marketing decision.
Here is an opportunity to demonstrate your performance art for an audience of genuinely interested people. Don't neglect the fact that you are Live Performance Artist. Here you are 'live and in the flesh' ready to have potential clients experience exactly what it is you do. You don't have to try to explain your show over the phone, send them to your website, show them a youtube clip or just give them a business card - you can amaze them personally right then and there.
As magicians we have the distinct advantage over other professions in that people are fascinated by what we do and with a little effort we can make a powerful impact performing informal and impromptu (seeming!) magic.
If you ever use the excuse that you "don't have anything with you" I would strongly suggest you take up a different vocation or hobby. You have no right to call yourself a magician. Put some thought, effort and time into what you would and should perform in these instances. Be prepared. You will be rewarded for your efforts.