Depending on the setting and how I'm prepared I've found several approaches that work great, my most favorite is what I call "The Searcher".
Essentially I move about an area, circling my target group, acting like I'm looking for something. It rarely fails that someone in the group asks "What are you looking for?" at which point I can move into action.
More than not I use this as a way to move into sponge balls; I hold my left hand so that the thumb & index form a "C" and ask "Have you seen any of these?" . . . obviously there's nothing there and as a second thought I "rub" the "C" with my right hand and say "Here, let me dust that off so you can see it better" and voila! A single red sponge appears. . . obviously I move into my spiel from that point and everything unfolds as it was intended.
When I worked the Playboy clubs of old I'd literally ask people to move over as I looked under the table, explaining that people leave some of the darnedest things stuck under the table. When I stood back up I'd be holding a small Bird Cage (ala Blackstone) and act as if I found what I was looking for . . . the appearance of the cage serves as an excellent primer to things and prepares folks for what's about to happen. But this particular bit of material also pulls other people into what you're doing because it's big enough to be seen by everyone in the room. HOWEVER, this isn't a routine I'd recommend for anyone not well versed in said effect, it can have it's headaches.
Albert Goshman used to approach people by saying, "Allow me to magish for you. . ."
There is the classic approach of doing "The worlds most amazing Card/Coin/Dice effect" such as Bill Malone and others have used for a very long time.
I've seen guys quietly draw attention to them by standing off to the side and moving into a kind of "Mime" routine like tearing up a newspaper as they seem to be reading it . . . making the tears loud and obvious, only to leave a restore paper on the table for someone to find as they go to throw it away. . . or else they themselves do the opening & reveal, leaving the restored paper on the table for people to look at. . . similarly, any liquid in a newspaper will work (or the reverse, liquid from the paper, magazine or whatever).
Be seen playing with a coin or ball (walking them over your fingers and some basic manipulation) will almost always attract people. Let the them ask or simply escalate what you are doing as interest and group size begin to obviously grow.
As corny as it sounds, the classic "coin behind the ear" still works; especially if you can turn it into more!
The list is quite extensive once you sit down with paper & pen and start working the problem and how to overcome the obstacles.
