I must preface this by saying that as a married man I don't, as rule, flirt.
In NYC, the slightest interest you showed in another person would properly be construed as either A) flirting or B) confirmation that you're not from these here parts. For example, if you go to a coffee shop, a typical, non-flirtacious interaction would be as follows:
You (standing in front of the counter waiting to be served: ....
Barista (standing behind the counter, looking bored and not showing any real interest in you): ...
You (giving a clear motion with your head that your attention has now been turned from the menu to the barista): ...
Barista (spoken with a carefree inflection): Hey.
You: Hi. Could I get a large soy latte?
Barista (nodding slightly and walking to espresso machine): Yup.
This, mind you, is an example of a rather friendly barista.
The following is an example of an extremely flirtacious and almost aggressive barista:
You (standing in front of the counter waiting to be served): ....
Barista (seeing you she/he shows a hint of a smile): How's it going?
You: Not bad.
You: Could I get a large soy latte?
Barista (nods slightly and walks to espresso machine): Yup.
In the above situation, the barista has made it clear that she/he is in to you. However, I have found that in Portland it is common for a barista to ask you all sorts of questions about yourself with real interest and...a full-on smile! At first I thought I was being flirted with ceaselessly. But then I figured out that it's the near excessive niceness of the people in Portland that causes baristas, and everyone else here, to act this way. Which is fine. Until you start wondering about how actually to flirt with someone. It was so easy in NYC. You could just ask how someone was doing with interest and you'd have made it obvious what you were thinking. Here, that's common practice with everyone you meet any where. I began to think you had to ask really probing questions or be super forward in order to make your intentions clear. However, my new theory is that you have to show some disinterest. If you act cool and distant you may seem intriguing to someone who has learned to be open and kind at all times. I hope someone can try it out and get back to me on how it works.