That level of trust does not develop over night however, you must work at it. And, trust that has been built over time can be gone in a heartbeat if one of you starts doing stupid things to hurt your relationship.
An investment in relationships does not have anything to do with money. The investment comes from being there for someone when they need you and being reliable even when they do not. The best way for you to accomplish this is to do what you say you are going to do when you say you are going to do it. This is a simple concept. When the small things are taken care of then you can move on to bigger things.
If she calls you up and says she needs you to pick her up from work or where ever and for you to be there at a certain time then make sure you are waiting for her when she comes out. Do not make her wait for you. She will be tired and want nothing more than to see your smiling face at the end of her day.
Another way to earn her trust is to confide in her about things that bother you. Confide in her about things that interest you. The two of you will feel closer than ever before and this will show her that you trust her with some of the big stuff. If she confides something to you earn more trust by keeping it to yourself, do not spread what she tells you all over town.
Show some vulnerability sometimes. No one is as strong as they need to be all the time. Let her know you need her to be there for you when you are feeling down and just want to talk things out. She will feel that you need her when you do this. Try to come out of your comfort zone and let her inside for just a moment.
Do things to make her feel important in your world. If she wants to go see a chick-flick then take her and try to have fun. On the flip side she should be interested in doing the same for you. Be careful here though, because you might get caught up in what she wants and neglect your own stuff. Do not do that to her and she will not do that to you.
And last but not least, carve out some time for each other. Life can get in the way and with this being the technology age that it is you can get lost in all the text messages and e-mails and almost forget to see each other everyday. Making an investment in relationships is just this simple.