this is my routine and it works very well. Approach her and just ask the basics her name, skool/work etc. all the while gettin close to her (rubbin shoulders, look at her eyes) wtver she says u say u do too. Like if she goes to some skool, say u go there or u have a frend there or som thing. As soon as convo PEAKS (shes interested) LEAVE, tell her u gotta take care of somting and ull be back for her. Come back in abt 20 min and since u built a rapport she'll be REAL happy to see u and say some crap like u gotta go but take her number down and your good 2 go.
this technique is harder if u cant find the girl after 20 min, thats the only drawback i have. also before u leave the first time tell her somtin like "i gotta find my frend, but wen i come back u owe me a drink/dance" she wont say no trust me.