Here's a suggestion from a women's magazine; look for 3 seconds, look away for a moment, look back for a moment as if you liked what you saw so much that you had to double-check it, and then look away for a few minutes before checking back. If you can't manage a natural smile, just turn up the corners of your mouth a little, so she knows you're an interested suitor and not a psycho scoping out a potential victim.
If you want to do a body-language psych on her, use "mirroring," which means copying the way she has the parts of her body posed; you don't want to look like a little kid playing copy-cat, so do it slowly and gradually, as if it were part of the natural shifting of your body.
Why does this work? Take a look at happy couples and good frinds talking together, and look at their body postures; they'll generally be VERY similar, even identical. We subconsciously mirror the poses of people we like, and we can use this to "trick" people into subconsciously seeing US as people THEY like. (This will also work when you're making new friends, during job interviews.... any time you want to nudge people into liking you.)
