Be honest. Act like yourself. If there are parts of your personality that you think she will find distasteful, don't shield her from them or your girlfriend will sense that you are hiding parts of yourself from her and will become less attracted to you because of it. Be humble. Let her know that your are working on improving in some areas.
Surprise her with romantic gestures. Not necessarily the traditional red roses or massive stuffed animals, but gifts and surprises that clearly show you think she's special and you know what she likes. A sweet note or unexpectedly making her dinner will go a long way to remind your girlfriend that she has a nice boyfriend.
Listen to her. When you ask your girlfriend how her day has been, listen to what she says rather than just tuning out. Let her know that her thoughts matter to you.
Give your girlfriend space. Texting her nonstop and insisting on knowing her whereabouts at all times will make her grow tired of you. Let her come to you sometimes and don't worry if you haven't heard from her in a few hours. Trust her.
Act nicely to everyone. This means not having a temper tantrum when your taxi is late, not starting fights when you're out at night and generally being a pleasant person to be around. Show her you respect others and value how you communicate with them.
Remember important dates. Never forget your girlfriend's birthday, the various anniversaries of your relationship or any dates that are important to her. Remember the date of her big test or performance review at work and her parents' anniversary. Write them down on your calendar and don't forget to let her know you remembered first thing on that day. Make reservations well in advance or buy tickets to a concert or show you know she'll want to see.
Understand that she'll have bad days. Everyone has moods, disappointments and setbacks. Don't let hers make any difference in your thoughtful treatment of her. Let your girlfriend know she doesn't always have to be "on" with you and that you care about how she feels.
Do things your girlfriend wants to do even if you don't. Let her pick the movie at least half of the time. Visit her family, wash the dishes and try that new food she's into.
Never speak badly about your girlfriend to your friends or anyone else. Even if you're angry, tired, having a bad week or annoyed with her, keep your business between the two of you. At the same time, never miss a chance to say something nice about her to others.