You don't have to spit all your game and get their number in the first 30 second period. If it feels unnatural it probably is. Do it in segments. Built rapport in the first encounter and get her number on the second or third visit. You need to make a solid first impression the first time you go in. When you visit the second time quiz her to see if she remembers your name or your drink order; if she remembers either one or even both after just one visit that's a major IOI. Even if she does remember everything, NEG her (in a playful teasing fashion) that she got it wrong. I'd say three visits max on asking for the number. It gives you a timeline and keeps you from getting boring or simply becoming a friendly regular customer.
~Antithesis