First of all I use to add random hot girl which are usually friend with somebody I know already.
Almost everytime she come up with a question like. Do I know you. At this point, I answer something like :
-I dont think so, and I know its me that had add you, I'm sorry :S..
All the time I receive an answer like.. haa its ok
Then you start,
And i'm Lucky You're cute ..
They almost say all the time thank you or a thing like you too .
After all these stuff you start to talk about you and ask questions about her.
Five or ten minutes later tell her you have to go so she can give you her phone # and you'll continue this conversation by text.
But a lot of time when you tell her you have to go.. she automatically give you her number.