1) *Stick out your phone , hand it to them * say : PUT YOUR NUMBER THERE
2) Give them a piece of paper and pen and say to them be creative . They'll think for a sec than say to them : put your number there are draw a little picture of yourself

(credit:STYLE)
3) I gotta get back to my friends , gimme your phone and we'll contact

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4) Take their phone , dial your number on their phone and call it just to get their number . Than say , OK , i have your number , you have mine .
Always command . Never ask for it . Dont say CAN I HAVE YOUR NUMBER or something like that . Just command them .