Use a simple observation-based opener: "Hey, I noticed you across the room and I thought I'd come say hi. My name is..." It signals you've noticed her, creates a reciprocal feeling, and sets a positive tone. Keep it brief and genuine, then move to a pre-planned topic.
So what is a simple way you could get close enough to talk to her? This again applies whether you already know her or have not ever met her before. In some ways it can be easier if you already know her. You know, just have a reason to just walk over and talk to her. But at the same time, it can be really simple even if you've never met her before. For instance, a really awesome go-to line is, "Hey, I noticed you across the room and I thought I'd come say hi. My name is..." and then you continue to talk with whatever you decided you'd talk about before you walked over. The fact is something simple like that makes her go, "Oh, he noticed me just like I noticed him. That's really cute. And then he came over and talked to me." And it's just, it's absolutely adorable. Definitely a good way to start this out on the right foot.
