I used to ask this question a lot to myself and did a bit of research to be honest i didn't learn much at all, i mean if you've read the bible there a lot of different things on eye contact making it a much more obvious one. one thing i did learn though is that flirting doesn't have to be obvious just a mix of things adding up to be obvious
In my opinion everyone is a natural flirt, it's just about realizing when your doing it.
If you've got a chick laughing, your flirting
If you can safely use kino without a weird look, your flirting
I mean even smiling, your flirting
C&Fs = Flirting
Practice is what you need a lot in this one, and the first step to practice is knowing that your doing it
Trust me you'll be talking to a chick one day and it will just click and you will realize that your flirting and see is enjoying it, it's just you're to busy trying to get her to impress her you don't realize.
Flirting is probably the easiest part of game once you've figured out what your doing and when, the hardest part is making the jump from flirting to something more, but step my step brother
Also, it's really hard to flirt with see isn't interested, learn this to, and just don't bother with those girls, In my experience the main reason you're having problems flirting is because she doesn't want to