In my opinion "You wanna see a magic trick?" and "You wanna see something strange?" are some of the worst things you can say. They may work for other people, but it never works for me.
"You wanna see a magic trick?"
It makes you sound like a little kid whose uncle just taught him a card trick.
"You wanna see something strange?"
If a random person on the street asked
you if you wanted to see something strange, would you really want to see it?
I usually go for something along the lines of:
"Hi, my name's David, I'm a magician, and I'm out here shooting a video for the internet. I was wondering if you could help me out for a quick second, I think you'll enjoy it."
If I'm not filming:
"Hi, I'm David, I'm a magician. I was wondering if I could show you something, it'll only take a second, and I think you'll enjoy it."
Dylan P. wrote a great guide right here on the forums:
http://forums.theory11.com/showthread.php?t=7383&highlight=street+magic+guide+approaching+people
I also recommend choosing you approach wisely. Don't walk up to anyone who looks like they're busy. Look for someone who looks they are having/want to have a good time and don't have much going on. I've noticed it's easier to perform for one person, but you'll get much better reactions if you perform for a group of people who know each other.