DonGorgon is directly on point.
I actually had to learn this lesson the hard way. For some girls, it is entirely possible to get them to cheat on/leave their boyfriends for you. All you have to do is make yourself look like the better choice.
Things to keep in mind:
1.) Don't date them. Or get attached to them. You will be setting yourself up for failure. If she cheated on her boyfriend to be with you, what makes you think she won't cheat on you with somebody else? Are you really just that much of a smooth operator? Remember what Ludacris said; "You can't turn a ho in to a house wife."
2.) It is a ****ty thing to do. If you wind up getting punched in your mouth, you had that **** coming. Especially if you operate through deceit and the power of shiest. If I found out that somebody was lying about me to my lady friend in order to get in her pants, or that the same somebody actually got away with it and banged her, I would tell the woman to kick rocks, and shove my foot up that douche-bags ass.
P.S. Dudes get serious over their *****. I've beaten ass before over tang, and I've seen people do some seriously evil **** over the vagina. Just remember that the vag is a beautiful thing, but with it comes the necessity of self-control and maturity. Tang has corrupted better men than you and I.