You're performing at a Bar Mitzvah when all of a sudden, the area around you seems to brighten. Is it a spotlight? Is it the Magic Signal? Is it the first sign of an alien abduction? Actually, it's the bright light from the camera man who just began shooting footage of your performance.
This is important footage - if you can get this footage on to the video of the event and the footage is good, there's a lasting testiment to your skill that your clients will see every time they revisit the memories of their event by watching the video. It's also a slick way to promote yourself - your client will probably be showing this video to all of his or her friends and family. So what can you do about it?
Talk to the camera man before the party. If you can do this without interrupting his or her pre-party set up (if there's anything worse than not talking to him or her, it's pissing him or her off), you can communicate (a) that you'll be footage worthy and (b) that he or she will need to film from certain angles to capture all of the magic. Also, rather than be on the defensive ("Oh shit! The camera man's here - have to make this minute or two count!"), be on the offensive ("[Name of camera guy], come here! This is so cool that you'll be glad you captured it on film!"). You should also consider coaching your audience if you see the camera guy coming by: "Hey, it looks like the camera man is coming by, so I have an idea. Let's make everybody else jealous that they're not here right now - when I do this [raise a card or a finger in the or do any innocuous action], everybody will cheer like this was the most amazing thing you have ever, ever seen in your life. Sound like a plan?" Just make sure that the audience will see this as something they're doing for themselves and not for you.
Be conscious of the camera man and reap the benefits of knowing about the ways you can use him or her to your advantage.