I'm naturally a very smiley, easy to laugh kind of guy. Lately one of my areas of focus has been to stop smiling, and instead smirk. Do this for a day and the difference is night and day. Girls feel intimidated but also intrigued by a smirk. You cannot smirk around a girl without her asking you what you're thinking, why you're looking at her like that, what's so funny, etc. It puts the fear in them because they immediately think that you're laughing AT them. Which is an EXCELLENT thing for them to be thinking.
Smiling is social and something people do in groups. A smirk is private. A smirk says that you're thinking of something that's amusing to you, but you don't feel the need to share it. You are perfectly happy with your funny thought and you won't be any happier by sharing it with someone else. A smirk is very unneedy. You cannot appear needy and smirk at the same time.
A smirk is confident. People without confidence do not smirk. According to the free online dictionary "smirk" is defined: An affected, often offensively self-satisfied smile. Smirk for the tingle.