Dude, it is the same as approaching women, u see a group that is out there having fun, approach, no but's. if they say no, its fine, next. rapid fire mode is the best way to practise.
Everyone that says "perhaps i should wait for the correct audience" is bull, it is just a number's game, about half of the people will always say yes if u go up and crack a smile and be friendly, u got nothing to lose anywayz. remember this, YOU are doing them a favor in showing them something cool and something that they will remember the same day next year. if they say no it is their lost, and hej, proper dressing helps !!
Personally i rarely get people to say NO when i ask if they want to see some magic, that simply comes from experiance i guess, so here are some pointers:
NOTE: this is basic human communication skills, more social activity instead of sitting in front of the PC !!! (i believe david stone's DVD teaches alot of this)
#1, with a group of people, know how to find the key person, that is the person where everyone is paying more attention to, and listen more to, it is important to notice that person especially in a restaurant or bar situation, once u find the person, approach and get the aknowledgement from that person, and the rest of the group will always agree.
#2, when people are in a deep conversation or doing some stuff, best way is to walk up, stand next to them, wait for them to aknowledge your presence, then proceed with the introduction. it is very rude to interrupt people especially when u are a stranger.
#3, keep the introduction short and to the point, as u really dont want to waste time on people that doesnt want to see your magic. i do advice against starting out the introduction with any production effect of the sort as if they gonna say no to your performance, they will still most likely say no after what ever u did, and if they do say yes, it will just be bunch bad audience anyway.
#4, when family is walking around, track the attention of the kids, no parent say no when their kids is loving it, and hej, it is significantly easier to entertain kids not because they are less smart, just they have a more positive and curious attitude towards life really. we should all be kids more often if u ask me.
#5, this is i believe the most important thing that u will need to know, u CARE TOO MUCH of what other people think of you !!, stop that, "I could approach that group of people BUT they might be talkign something more important, BUT they might be going somewhere, BUT they might be blabla, WHAT IF they are blabla, seriously, WHAT DOES IT MATTER?? just go up and ask.
hopefully those helps.