I'll only add to what Schism169 said, because he's right on.
No one can tell you how to be natural. That is something you have to figure out for yourself because what's natural for you is different for what's natural for me and so on.
As far as approaching spectators in a casual environment, i.e on the street, in the mall, places like that, saying, "Hi, do you wanna see a card trick?" works brilliantly, or "Can I show you something cool?" works just as well. One time I was at a restaurant, eating outside, and I wanted to do a trick for some people at a nearby table. I waked up to them and said, "Hi, I'm just a random guy on a street with a deck of cards. Do you wanna see a card trick?" It got a laugh, and they wanted to see a trick. I ended up doing three tricks and they loved it. Note, we were outdoors and anyone could have walked up. This wasn't a fenced in eating area. If it had been a fenced in eating area, I would not have done that.
Also, make sure they have a minute or two to watch before you launch into your 7 phase ACR that takes a whole 5 minutes to complete. (That was a joke, btw.)
Other than that, just have fun! If you're having fun, the people will have fun. I make jokes all the time based on what my audience is saying and doing. That's my "Character," and it fits me very well. It also let's the audience know that I'm paying attention to what's going on.