As far as visuals go, I'd say just do the more subtle stuff - don't exaggerate the curves on her body, but maybe the eyelashes a bit, that kind of thing. Other than that, I guess what you're trying to do will mostly have to be accomplished through dialogue, and subtle characterization is, well, yeah, it's hard.
Exactly good dialogue can say more about a character than the way you draw them. I seriously hate to tute my own horn as they say but dialogue is actually my strongest area if you want to skim over some of my stuff you might get an idea. Look for my character Lilith in Wakon Yosai. Shes the one with angel wings. Man... If I come off like a dick for sayin this please forgive me ...
Flirting has to deal with charisma and high charisma is more about your force of persoanlity than anything else.
It's hard to explain because it's something you have to experience, but certain women who look "plain" by the modern media's standards (I.E. if you don't have big breasts, blonde hair, and huge thighs) have a way of exerting their personality through subtle body language and you can mimic this in your comics.
First off, experiment with lettering. Adding a little heart after sentences help the reader pick up that this character likes to flirt. For example "These bags are heavy. Would you mind carrying them, big man (heart)?"
Next is keeping the character focused on the panel. A character who speaks while the panel is focused directly on their face is neutral. But if you turn that woman's face to the side, have her hair cover one eye, and pucker her lips out a little bit you suddenly add a slight bit of mystery and sleek sexyness in her pose. Putting one hand on a hip and slightly thrusting out is common body language that dictates interest or attraction.
Ugh, I hope I don't sound like too much of a nerd but I study social interaction as a side hobby. It's interesting how certain body movements come natural to us and you can read someones emotions through how they move just as easily as you can through words.
Look at it this way... A great character is somebody your audience can relate to. In this case think about the real people you know is there a flirty girl in your RL? What would she say if she was in the situation your writing? What is it about her thats so sexy in the first place.... You can create any character this way.