1. You have to understand that sometimes women just won't like you for their own personal reasons.
2. You have to build value to get them intrigued, a simple approach and asking for a number doesn't work with quality women.
3. Make the approach seem as little like an approach as possible, lower her guard.
4. Before you ask for a number, engage in a normal conversation and listen or look for indicators that show interest. Once you've seen a couple, and the awkwardness is gone from the initial approach, then ask for a number. (i.e. they ask you questions like your name if you didn't give it, where you're from, about your career etc...) "So, what actions can we take to continue this conversation later,' she should instantly offer her number, if not just hand her your phone with the dial pad showing.
5. If you feel confident she's interested because of the indicators, wait a couple days at least and call her, have a plan set and say it in a confident way, don't give her a chance to say no. (i.e. 'Hey, I have some tickets for us to go to blah blah on XX date, make sure you're free')