Be honest with your girlfriend, don't let her doubt your words. If you lied and truth "came" to your favorite, be sure to explain the reason for such behavior and really nothing to be honest.
Try to always apologize for their wrong deeds. Learn to do it right, don't dry using the phrase "Well, excuse me. I won't do that again!"
In an argument, don't yell at her, do not show aggression, and especially don't raise your hands. You are mentally normal person and is able to solve everything in soft colors. During the showdown, listen to what she says, then analyze and select future equipment behavior.
Responsible for his words, that is, if you something promised – will definitely do. Remember, you must be reliable! If you doubt you'll be able to perform, it is better not to take the case.
Do not wear a "mask". Be yourself, no need to turn into an actor, because life is not a game. Note that mask sooner or later will fall. And then what? Then the girl may simply be angry for this performance and will cease to trust you at all, which in turn will lead to the breakdown of relationships.
Communicate more with her, be an open person, do not turn, as this will induce her to believe that you are hiding something. Tell her about your life, ask how is her way of life. You need to know more about her, including talking about myself.
If you for some reason lost confidence, admit your mistake. She explained why this happened. Tell me how you cherish it and don't want to lose. The path to gaining a trusting relationship is sometimes long and difficult, if you really cherish the beloved let it in you never questioned.