The majority of men don't know how to get a number from a woman they are attracted to. Here is how to get the digits with a minimum of stress.
Release your anxiety.
The first step to success is to relax. Being nervous will project a bad image, and she will pick up on it right away. Breathe deeply for a minute, shake off the tension, and make your approach.
Break the ice.
Find something outstanding about her outfit - whether it be stripes, a large buckle, high heels - and ask her about it in a condescending fashion.
"Is it hard to walk in stilts like those?"
"Is it true that stripes are slimming?"
While this is insulting, the fact is, it works. She has little choice but to pay attention to you, to find out if you are being a jerk on purpose or if you are just an idiot. This is a question she will forget about soon, so it makes no difference.
Let her talk.
She will try to pry answers out of you. It is your job to redirect the conversation back to her. After letting her talk for a few minutes, casually wander off and mingle with others in the crowd. Women hate being ignored, resent men who do it, and cannot forget a man who is confident to the point of being a jerk.
Wait for her to come to you.
You have planted the seed in her mind, now you must let it grow. When the fruit of your labor is ripe, she will come to you with an excuse for striking up a conversation. Tell her you would like to continue your talk later, and ask for her number. Not only has she forgotten what a bastard you are, she has also made herself available to you for later.