Priceless
I have done this unconsciously before. This post has reminded me why this has a positive affect now.
Time and time again I have seen guys wonder, "I'll do x to impress her, I'll buy y to impress her".
Fact of the matter they should also focus equally on what can she do for them. In the real world the more you invest in something, the more you value it. This is also true in relationships. In the back of her head she will add up all the effort she has invested and it will help her justify how great you are. "I've done this, this, this, this and this, mmmm I've done a lot for him, he must be special, why else would I do it!"
If she has invested a lot emotionally and physically in you, it makes it very hard for her to drop you when things get tough. She doesn't want to lose her investment without good reason.
As stated she needs to rewarded, otherwise she will think its a bum investment. So she does more for you, you reward her, it justifys her beliefs, she continues, you reward her etc
In the beginning be comfortable enough to ask them for little things, or you can trade favors. At least she knows that you're not going to give everything away.
Goes hand in hand with the idea of challenge.