Females attempt to flirt by giving out vague signals that may or may not indicate interest. They are like IOIs except that they are intentional. Flirting is the first stage of human courtship.
I swear all girls must have be told at some point "if a guy you like looks at you, make sure you smile." I cannot count the number of times I have made eye contact with a girl and had her respond with what is clearly a forced and awkward smile. So here's the advice: if it doesn't come naturally, don't do it. A forced smile does nothing to improve your attractiveness, and only tells the guy that you are nervous or awkward. While a smile might encourage a guy that needs some reassurance, I guarantee that it will not make or break his decision if it isn't natural. Forced smiles are very easy to recognize, even for men. Prolonged eye contact alone should suffice.