For all of the morally bankrupt Lotharios who requested advice on stealing girls from their loving boyfriends, this one's for you.
1. It doesn't matter whether or not your girl's boyfriend is there when you meet her; you are not allowed to openly flirt. Instead of flirty, you just have to be really awesome. (I know, I know, easier said than done, but I've got you covered.) What I mean by "awesome" is that you have to be outgoing enough to attract her attention so at the end of the night, when she's happily snuggling with her boyfriend, she'll still sort of be chuckling about something you said.
2. Ok, now she thinks you're cool, but unless you have thoroughly intoxicating pheromones, she's still not thinking of ditching her loyal boyfriend for you. So, you've got to try and fight your way in through the Friend Door. Relax, you don't have to go shopping or put on make-up with her; you just have to be the person she goes to when she wants to complain about her boyfriend. In order to get to this friendship place, it helps to establish some base-line thing that you and your girl have in common. Might I suggest being a super-fan of a show she loves and her boyfriend doesn't? That "It's Always Sunny in Philadelphia" half-hour is yours.
3. When you talk about her relationship you have to always make it so that she says the critical stuff- she won't believe anything's wrong till she says it herself.
(A sample conversation:
You: My father just has this way of making every situation about himself. Like, he will even compete with me about who has had the sh*ttier day. Do you know what I mean?
Girl: Absolutely. (Insert stupid boyfriend's name here) does that kind of thing all the time. Like the other day I was bitching about this English paper I have to write and he was all like, "You don't even know what hard is. I have an Economics test next week that would literally singe your brain."
You: Ha (laugh at her impression of her boyfriend. Girls are horrible at impressions, but go with it). That's just like my dad, and the worst part is that my dad is just so patronizing about it. Sometimes I feel like he doesn't respect me.
Girl: God, I totally know what you mean.)
4. Brush your teeth, put on a clean pair of underwear, and wait by the phone. Your girl is now out screaming at her boyfriend for not respecting her because he told her that his teacher was more of a dick than hers. Your girl is going to run crying from him, call you, and ask if she can come over. By this point you won't even have to try and put on any moves because she's yours for the taking. After you take her though, you have to be the one to freak out and feel guilty. She'll comfort you and decide that things really are over with the boyfriend and that, you know what, she doesn't even feel guilty about it.
5. Congratulations. Now you've got a girl whom you will in no way be able to trust because she is a cheating whore. You asked for it.