For that matter a lot of my guy friends want to know... How do I get a girl to know I'm interested?
Girls: If you're interested in a guy, and you've made eye contact with him; smile.
It's that simple.
There's actually more to it, but the odds of someone approaching an unfriendly looking person are low in comparison to the odds of a guy approaching a girl who looks like she's friendly and having a good time.
The next step? Look away. Don't stare the guy down like a piece of medium rare steak in a world full of salads and hot dogs. When you do eventually look back (could be a minute later or 10 seconds after the original eye contact) smile at him and then look away.
This shows interest.
Guys: If a girl makes eye contact and smiles. I'm talking a genuine smile, not that grimace-y-oh-my-god-why-is-he-looking-at-me-smile. Then it would be a safe assumption if you catch her eyes a few moments later that she is relatively attracted to you. So, if you're interested in her... SMILE BACK!
I can't tell you how many times I have been places and played the look, smile, look-away, look, smile card. It works. This is an actual technique that when deployed properly and received by a receptive member of the other sex will never fail...
but there's a catch.
After you've done this smile look smile interchange. You've got to go talk to them. You can't just wait for them to come to you. Either party needs to make the initiative to actually stand up from their table and walk towards the other party.
If you're shy... there are other ways to do this...
Let's give an example!
Once upon a time, many years ago, I saw a fairly attractive guy walking around my campus. My friends knew him, but they didn't know him well enough to introduce me to him, no matter how hard I asked. So, one day, I began catching his eye and smiling at him, whenever I'd see him and sure enough he would do the exact same thing when he saw me. It became very apparent very quickly that we were attracted to one another. Problem was... we didn't know each other... so one day we happened to be at the same St. Patrick's Day party hosted by a mutual friend. He found me on Facebook through that same friend since he found out my name, shot me a message and shortly thereafter we began a very lovely dating relationship.
This stuff works.
I'm not saying it's going to get you a boyfriend/girlfriend/husband... that I know of, but I can guarantee that it's a step in the right direction towards meeting new people.