I've been in several serious relationships (Yes, i try being mostly monogamous) of a year or more and have aquired a few pointers.
- If you get needy she will lose interest. Try refusing to bring her when you go out with friends, and when you get home talk a little about how this ugly chick tried to hook up with you. Normally, guys don't talk to their girlfriends about getting approched by other women, but trust me, it's a good idea. It has to be done right though; casual, disinterested, and a good amount of putting the other girl down. It doesn't work if you elaborate on her great looks and wit, then it'll just seem like bragging. If you "accepted" her approach, well, just don't give her that part of the story unless she's really loosing interest.
- When you do have sex, try telling her you're not going to finish, then try just giving her foreplay and oral until she's turned on good. Then stop and start reading a book instead. She'll hate you, then probably jump your c... instantly. Push her off a few times before giving in. Or don't give in at all if you have that kind off self dicipline. I rarely do myself.
- Do a little flirting with her friends. Get "cought" in giving a hungry look etc. If they think you're attractive, your girlfriend will most likely agree (you know, social status and all that bs). Never be too nice and sweet, that's not interesting.
Of course you can always just dump her, but I prefer to get what i want instead of getting something else.