I find the serious attitude surrounding flirting in this thread to be a bit strange. I flirt all the time with everyone. It's just meant to be playful and not taken seriously. I think you can form better relationships with the people you are around if you flirt with them a bit. It seems like some people take flirting as a means to lead someone on and as just being some kind of game to toy with another person's emotions or some kind of malicious manipulation.
I think there's a time and place for everything. I don't mind mild flirtations but I am in a serious relationships so the flirting I do tends towards NOT being romantic or charged in that way at all. I still think you can be flirtatious and fun without there being a romantic and sexual component to it. It can just be as simple as innocent teasing and joking.
I have no problems being very direct when I need or want something but I find flirting as a whole to be pretty harmless. To me it is just a different layer and works in tandem with being charming and charismatic.