how about my girlfriend?

Well.. I'm not an expert in this.. or I might say I am low on practice, but I like to think I know the theory behind it.
Here it goes.
We all like to be cared for.. better yet.. invested in. If your girlfriend invests time in her for example putting on makeup is only to make you invest your time in her. Invest in her. That is all that is it about. So, appreciate this simple gesture, and even follow it. Us guys have to give something they like too. There is another catch to this. If you invest all of your time in her, she will back away as she will consider you a fool (and for good reasons). Here comes the thing about obligations. If you invest too much in her she will feel enslaved by the thought that she ows you something. Instead, you should try to focus on yourself. Invest in yourself so that she can appreciate your value and feel free to do so. Don't invest, let be invested in, give reasons for that. This way, while both of you investing in yourselves, will give the other the interest he/she needs for investing in himself/herself to rise to your level (it's self sustaining). That's why I consider competition a good thing.

Another thing is about expectations. If you already expect something, you are more likely to be disappointed. Try not to expect anything, and be surprised.
Being predictable is the worst thing for you. I've read an experiment done on pidgins that simulated what people feel when gambling, and the unpredictable event keep them asking for more. The same situation is with relationships. Some unexpected (sometimes contradictory) events may have the effect of cheering up the relationship. That's why playboys get all the "action". Because in their case, the confusing behavior is determined by not carrying, so, believing it or not, they are craving (without even knowing) for what they hate most.
But, how about good guys using dirty tricks in good intentions?

There is also the thing about liberty. Nobody likes to be owned.
I think a strong relationship is made in time (and from what you have learned by now you understand why). So, if you know each other for a long time, there is no need to worry about losing your pair, because you're what she's invested in. She will not give up so easily on that (if she is smart enough). If she does, well.. then she most likely doesn't know what she wants, and you should try to invest in some other person (this is rather hard to do, at least at first.. long subject, maybe a dedicated post later), or maybe give her some time to figure out (if you are strong enough.. this implies her full liberty). This is the thing about investments.. you have to know who to invest in.. you have to ask yourself whether your investment will pay off. Don't ever invest too much without getting feedback (I should read this too after I've written it). Sometimes it just doesn't work or doesn't work anymore. It may not even be you fault. It's just situations.

I know I have missed something, as we all do.. we are only human.
What I've said is valid both for men and women. It's what they look for in the other that makes the difference. How you choose to invest your time in yourself (I'm not going to help you here).
I made 3 simple rules that mostly came from the same movie:
1. give total liberty and accept nothing else.
2. be honest (this might not work), and have as little secrets as possible (the nature of these secrets is up to you - I would use them only to make surprises and provoke, for your mistakes at least give the other the right to judge the faith of your relationship, if you're afraid of the outcome, I'd say you're a coward).
3. don't mess up with the other's feelings. If you don't find any more interest, tell him/her that, at least to know what went wrong (this is in somewhat relation with the latter one).