Establish some commonality and try to get the number from that. If you ask for a number directly it sends up red flags. Sure you can do it, but it is harder, you have to have a lot more attraction. There is some truth in the statement, "It's easier to sleep with someone than get their number". First of all, realize that the number is not your goal. You want to hang out with her. One thing you could try is to ask her next time when her shift ends. Then make sure that the next time you come, that you come about 40 minutes before she gets off. That way you can talk to her, tell her about something you know she and you both enjoy doing which will be happening immediately after, and try to pull her directly from her job.
The other option is to establish a common interest, then tell her of some event which you are either hosting or going to, which she should totally come to. Once she agrees that she is interested in possibly coming (requires almost no commitment on her part), ask her "What's the best way of my getting in contact with you so I can send you the details". If you are lucky, and she is feeling comfortable, she will give you a number. You can reinforce this by pulling out your phone. If she starts to back pedal or hesitates, say "Do you have a Facebook or something?". That's less commitment and much more likely to get a response, yet it will get you much closer to the number.