Shezz said:
They're all based on a few basic principles. Basically, you want to create a VOID in her life by letting her see that her BF is not fulfilling her needs. Then you show her that you are precisely her-void-shaped and ****ing you would make everything alright.
Nice post and really good point above.
I'll go even further to say that it would be extremely effective if you can figure out exactly what that void is and, in an extremely round-about, unintentional way, demonstrate that you could fill that. You can't just say: "So what does your boyfriend lack." Then she replies, sensitivity. Then you just say: "I'm a sensitive guy, by the way." It doesn't work that way.
This whole thing is also going to depend on whether or not there is a possibility that she'll stray from him. Some women genuinely do hold true to their committment (believe it or not), and accept their man for his flaws and don't rely on him to fill her voids. But if there is even a hint that she's keeping her options open, you may be able to step in and make the steal.
The only thing left is to tread lightly. Stay away from any words or action that could cause her to step back and examine what she's doing...at least until she's roped.
And finally, make sure she knows that she can trust you. If she feels that she can trust you, then advancing things with you isn't as much of a threat to her since she figures that nobody will find out. One thing that might help her trust you is to share something that you keep a secret (hell it doesn't even have to be true), go into a small rant about how "you don't tell many people this but you feel like you can trust her" and share it.
It sounds gay but I can guarantee that if you try to walk into the situation like a bull in a china shop with the macho/alpha garbage that's encouraged on this board, she's going to close-up and shut you down... unless she'd already had plans to cheat.