I usually ask a girl if she has a cigarette, then when she gives me one, I look at her cigarette pack,
If she has a lot, then i usually don't mention them, i just keep the conversation and so on.
but if she has few, I smoke mine, while holding the conversation, and after taht, i take my pack, an give her two. i tell her that she owes me one, and that's why she sould give me her number.
if she has few cigarettes just say this: "Here are two cigarettes" *give her the cigs* "the real reason i asked for one, was to talk to you. You owe me a cig, so I'll have your number"
you have to say this like you are in control, like you are boss, and dominating. You have to say it with authority. if she smells just a little insecurity, she'll say: "don't boss me around" or somthing like that
well hope it helped

Good luck, have fun
