When you approach someone, you should ask them "Can I show you something?" and go along and perform your trick.
I have only been doing magic for about two weeks but i have about 5 tricks that i got good enough to do at local bars or partys.
When I ask, I just walk up to them and ask "can I show you something?"
you can take that in a perv way on here, but face to face you have to come off like your normal and you dont want to freak them out.
So smile or joke around if they look at you funny.
I have been going out and doing this for about 1 week now and I have walked up to about 20 people and never got turned down except once. And when you do it more it builds up confindence.
If you tell them what your going to do then they focus on the trick and you dont catch them off guard.
Just dont dress creepy and dont act weird(i have seen alot on you tube were kids dress creepy and act like there about to rob you, lol). Act natural and be yourself.
What I like to do is hang out somewhere with friends so people can see me laughing and carring on and they know im a normal person so when I approach they are not timid.
This is only my first post but i hoped i helped.