There are a lot of things that are fairly universal in this area: smiling, laughing, being energetic, talking to other people who are around, making eye contact.
Most dating and courtship studies show that men will typically only approach a woman if there is a moment where the two make lingering eye contact. On the other side of that, when men DO approach a woman when no eye contact has been made, or if the eye contact is fleeting, he will usually be shot down.
If there is a specific guy that you want to approach you, let him catch you looking. When he does catch you looking, don't look away, allow your eyes to lock for a moment. Chances are he'll come over and say hi.
Guys will look for any number of things that all tie back to one central question: "How likely is it that they're going to shoot me down?" It's a big risk that a guy is taking by approaching someone that he doesn't know and so I always tell guys to look for signs that they're into you, or at least open to talking.
If there's one thing for you to take away from this, it's to look like you're having a good time. No guy is ever going to approach somebody who looks like they're having a bad time unless he lost a bet or something. This is as simple as laughing, smiling, and talking to the people you're with, but it makes all of the difference. From there, you just make it known that you want him to approach you.
In the "pickup" industry, they call these Indicators of Interest. They work like a charm. Make eye contact when nobody else in your group is looking his way. Keep the eye contact for a few seconds before looking away, repeat. If you're in a crowded bar, brush lightly against him as you walk past. He'll notice you, and probably think it's a complete coincidence. Just remember that guys are horrible at picking up subtle signals and need a little bit more guidance than you think they ought to. When in doubt just ask yourself what you can do to make it clear that he's not likely to get shut down. With just a little practice, you should be able to have him approaching you in no time.