I was at a bar last night and it worked on 6/6 girls. I'm sure it isn't foolproof and depends how smooth/good looking you are, but here it is.
Walk up to a girl, buy her a drink, and make small talk for a few minutes. If she seems into it and you're making her smile/laugh, ask to draw her something. On a piece of paper/napkin, draw:
(_ _ _) _ _ _ -_ _ _ _
Then say the drawing came out well but you need her to finish it for you. Hand it to her upside down and give a witty smile. I got some great laughs and 6 numbers to choose from.
Happy hunting :p
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If you have to ask for their number, you're not doing it well enough.