I'm going to give you the ultimate cold approach that is infallible, unless she is just not interested at all.
You can thank me later once you used it at least 5 times!!
Here it is..........................
if you see a girl walking by you can say...
i will regret it later if i let you walk by me and I did not acknowledge your presence - i most likely will not see you again - would it be terribly impolite to ask for your name and phone number
if she doesnt want to give you her phone number then just dismiss yourself politely by saying...
"have a pleasant day and excuse my abrupt behavior but God designed us guys to do some crazy things" and then smile politely and walk away with your head high feeling like a man and proud that u have done your duty.
if she say yes i will give u my number (don't act startled - u deserved it) - then tell her "let me accomodate you with the means of transcription:...(a pen
)
then go through your pockets smoothly and not excitedly searching for the pen - dont find it too easily then it seems as though this was all planned and plotted as if u do it all the time and as if it was not an impromtu act - take a few seconds - and then have her write her number herself on a receipt or other paper that is not blank - again u dont want to make this looked planned - do not give her a lttle black book with other numbers in it - dont be arrogant be polite - then ask her "what would be the best time to reach you? " wait 2 or 3 days before u call - dont seem too eager
dismiss yourself with pleasantries - "have a pleasant day"
p.s. - dont forget to give her your name after she gives you hers.
also - if she is not walking by you then obviously u would have to use a different opening - such as, "i will regret it later if i left this place without acknowledging your presence...
hope this helps - let me know if it works for u - and of course make adjustments as u see fit and for what the situation allows. Use words you are comfortable with - add or take away as the situation allows.
good skill!!