Most PUA material teaches you how to approach, attract and close a girl in a nightclub setting. While the core concepts such as self-confidence, DHV'ing, push-pull etc. are useful in pretty much any "gaming" situation, the fact remains that attracting a girl and maintaining that attraction for a long period of time (over however long you retain an interest in her) are two different matters.

I've found that my relationships with the few girls that were at least as invested in me as I was in them (evidenced by things like always hitting me up to hang out or getting worried if I didn't respond) shared this common denominator: they had revealed intimate, "touchy" details of their personal lives to me. One girl told me about being molested when she was young and her close friends' drug addictions. Another told me more about her family problems than I really cared to hear. I think that telling these personal things to me made them way more invested in me, maybe because I now had some sort of special trust.

So my conclusion is this: a great technique for long-term game is to get the girl to reveal intimate details about herself to you. PUA led me to think that you don't want to ask too many questions, but in a more "get to really know each other" setting maybe this doesn't apply? Of course, you don't want to be that shoulder for her to cry on, so it's a fine line to walk. But done successfully, this is a very effective way to grow her attachment so that it's more of a two-way thing than you just chasing her around.

EDIT: Wow! Definitely did not expect such a turnout. Thanks everyone for the input, and apologies to those that thought I came across as too "cold" or "manipulative" or whatever. I meant for this to be a discussion of theory, not sentiment.