Just return the look for a second or two. Don't look away so quickly that she gets the impression that you don't like the fact that she's looking at you. Once you know she knows you've noticed her (that's a mouth-full), physically do something small to show your interest - maybe smile, wave or, I suppose, wink. Then break contact and maybe she'll come over to you.
Conversely, If she smiles/shows interest at you when you catch her eye, she might want you to approach her. For example, maybe she'll look down then smile back up.
Personally, winking puts me off. But that's just my opinion, other girls may have a different opinion on that.
Just some small sign of interest before breaking eye contact should get the message across adequately.