I'm quite witty, and pretty astute, or so I've been told (and now I sound like a cock!). But I've found it easy to bounce off people in social situations and that has always proved the best tool when talking to girls. There is no formula as everyone is different, but 'negging' does work, it was actually something I did just naturally because I like to take the piss and then heard about it through people reading pick up books. It is something that's hard to get right though as you have to be able to gauge how far to take it, different people have different levels of what offends them.
I got with my current gf through taking her seat in the pub, then taking the piss out of her sister (obviously in a playful way, not by being an arse). She's quite a reserved girl that would never have normally allowed to get 'chatted up' but I guess it was endearing.
Best advice to give is to be genuine, smile, and to throw yourself in to as many social situations as possible, being able to take charge and be confident is paramount. Maybe learn a shit joke, not too corny though.
Oh and LISTEN (x1000). Remember when you're talking to someone it's a conversation, even if you just want one thing out of it.
Finally don't put the pussy on a pedestal <- best advice I ever got.