Just go talk to her. Even if the circumstances aren't perfect, you like her, and that's the only prerequisite. If you're shaking with nervousness, then that's just one more reason why you need to approach her and get the practice. You'll be proud of yourself for doing it--no matter how it goes--and you'll be a little less nervous next time.

Be more social in general. Stop quietly seething about the stupid way strangers move their clumsy arms when they walk, and instead get interested in what everybody thinks and has to say. This mentality will make you more social, which will make you more interesting to--and better with--women.

Smile. Put yourself in a good mood before you approach her, and smile. Don't worry, you don't look like a psychopath with a dead-eyed stare as if you're hiding thoughts of filling up your basement's shackles with some fresh wrists and ankles. And even if you did look like that, it would better than looking serious or blank-faced.

If you open with something indirect (e.g. "Do you know which way 4th Street is?"), you're less likely to be outright rejected, but you'll have more trouble navigating into being able to pick her up. If you open with something direct (e.g. "This is really weird, but you smell like a girl I used to date."), you're more likely to get rejected, but you're also more likely to pick her up if her eyes don't roll up and into the atmosphere.

Talk slower. Guys are afraid that women will lose interest if they talk slowly, but the opposite is true: People lose interest if they have to think harder to understand all the words flying through their ears, and a fast speech pattern sounds unconfident. Only disregard this advice if you already talk like Droopy Dog.

Keep it light-hearted. She'll associate you with how she felt when she was with you. So save your opinions on "the decline of American spending power" for never.

Body language is important. Stand up straight. No fidgeting. Open up your language to invite people in. All this shows confidence. Talking with your hands shows passion and makes it more natural for you to touch her shoulder or back.

Don't ignore her friends. If you don't make them like you, those mother hens will love to cock-block you.

Get her away from her friends. The friends have to like you, but you gotta build a connection with this woman in particular, and the sheer presence of her friends hurts that goal. Make them like you, then make her ditch them for a few minutes.

Be curious about her, but riff about each of her answers to your questions. Don't reel off question after question or her answers will get shorter and shorter until all she says is "Bye."

Make eye contact. It conveys confidence. If you dart your eyes around or look down most of the time, she'll like you less. Averting eye contact is what you're supposed to do if you meet a chimp--don't treat her like that.

If you've been talking for 10 minutes or so, it's time to ask for her number. You probably could have asked after 2 good minutes, but after 10? Definitely. If you've got some time to spare, instead ask her to go do something right then, like grab a cup of coffee. If she can't, get her number for later.

So go get her. There's no way you'll be worse off than if you didn't talk to her, and you might end up being WAY better off.