Summarize:

Another way to get someone's attention is to make eye contact. This is particularly helpful if you see the girl somewhere like a bar or party. Brief eye contact can send the message that you have interest in this person and would like to talk.  Try to catch the girl's eyes. Glance at her when she's looking in your direction and wait for her to look back. Hold her gaze for about 4 seconds and then look away. Four seconds is long enough to establish a connection. Any shorter may be mistaken for accidental eye contact, and any longer may make things feel awkward. Flashing a winning smile at someone from across the room is a great way to get their attention. While holding the girl's eye contact, offer a quick and warm smile. This will convey you're paying attention to her. You can also try raising your eyebrows slightly while making eye contact. This is a flirtatious gesture that does not read as overtly sexual. This can show the girl you're a friendly, approachable person. She may be more likely to engage you with conversation later on if you raise your eyebrows during eye contact.
Make brief eye contact. Smile. Raise your eyebrows.