I felt that the best way is to be natural.
Personally i really dislike people who do everything just to get
someone's number or get somethings done thru all means of their
ability. I call them losers lah.
Leave it all to your choice.
But my advise is to get the number, if you got it. Great! 
But if failed, then don't force...but try to get something out of
it(info which can fall back on when you see her again) but don't
act like a fool.
If she's not interested, then forget it. Till the time she walks up
to you and say hi, maybe a smile is enough to initiate a chat once
again. 
