I've had so-so odds. Play the numbers game with cold approaches.
I'm seeing about 1 in 10, 1 in 15 is a good one. That's from the numbers I get, so I get 10-15 numbers and I get 1 good contact.
Matching up reasonably closely to Paul Janka's 11% rule - he sleeps with 11% of the girls who give him their number.
As far as numbers go, I probably do 5 approaches to get one number. I don't push very hard, though, I'll usually just sideswipe a comment and start a small conversation, and if I see interest in pursuing the conversation, I do. If I'm finding I have to ramble a bit, I just bail, which probably knocks out a chunk of people who I could get numbers from but just weren't breaking out of their comfort zones as easily as others.
As far as how to do it? I go for situational openers, I just spout out the first thing I think of. Still learning all that. Gets better/easier each time. Make a comment about their phone, I make a comment about them being anti-social and texting (easy one, but I bail a lot on this - but since it's easy, it works in situations where I can't think of something else). Seriously, just walking up to them, walking by them, saying something - you might get a few weird looks the first few times, but you'll quickly learn what boundaries you shouldn't shatter.
Should ask Demonpenz on that. He's pro with approaches.