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.