I think that should always be done afterwards. You'll have effectively auditioned for them with your performance, but you don't want to be blatantly pushing your services during your act, unless you can somehow make a (good) joke out of it. Otherwise it just lacks class!
If you get a chance to mingle with the audience beforehand, it might be worth scouting out any high-rollers and introducing yourself, and then tell them that you'll show them something special after the show. Then meet them again afterwards, show them something that'll blow their minds, and leave them your business card, making it clear that they can engage you for parties, or whatever functions you do.