There are two approaches that tend to work well, and they are oposites of each other so what ever you feel suits your style I'd say go for it.
Approach #1: The Assumptive Approach
*Walk up to the mark*
"Hey! Check this out... here take a card"
Psychology: Get them involved in doing something with a polite command. We are used to getting commands all day long from various people for various reasons and as long as it's given with an authoritative tone, without being rude or too demanding, most people will just go along with it. Basic NLP stuff.
Approach #2: The Basic Introduction
*Walk up to the mark*
"Hello! My name is <insert name here> and I've been asked by management to entertain you with a little bit of magic. Please, take a card..." Or what ever your first effect is.
Psychology: Establishing who and what you are is a good thing. It also makes the spectators feel more special since management cares enough about them to send you over. You still get into the magic by asking them to perform a command such as picking a card because you never want to give them an opportunity to easily say "no".
The Rejection:
Asking a spectator if they would like to see a magic trick will more often than not result in them saying no to you. They may see you as intrusive, they may think there's a scam or some kind of financial commitment hidden with your generosity to perform. What ever the reason that goes thru their head they weren't prepared to see magic, didn't know they would see magic, and here you are asking them if they want to see something. It's not surprising that their immediate reaction, before they really even think about it, is a knee jerked "no."
This is why you never give them the option to say no. You can always start out with a soft command, such as "Here pick a card." If they really don't want to see magic, they will interrupt you and shoot you down. That's when you know for certain that they just don't want to be bothered. By not giving them a question they can easily say no to it reduces the chance for an off the cuff unqualified reply. More often than not, you'll have more success performing for strangers when you go this route.
