Gotcha,
For me, the dynamics of picking up women or even socializing with women, is a lot different than maintaining a relationship. The techniques used to attract a woman aren't the same as keeping her attracted.
There's a lot more to keeping a girl interested than just investment, but since you're asking about investment, I'll write about it
Investment, in relationships, isn't necessarily about getting her to do things for you, but it's more about not giving in to every demand she makes. Like "Honey, can you get me a glass of water?" Don't get up and get it for her, have her do it for herself, but if you're already up to get water, then do so for her as well.
That way she's investing her time into something that you could have done for her anyway.
As a result, she's putting in more effort than you are, and is associating that effort with her feelings for you

(A little cynical but it works)
Now, if you're trying to keep a girl interested and attracted, then make it known, or rather shown that other girls are interested in you. If she knows you have other options, she'll invest a lot more time and effort into you to keep your attention, and to get you to keep your eyes on her and not other women.
Pre-selection, and other options in relationships is the ultimate form of getting investment
