In modern society, many people take others for granted and do not treat people the way they deserve to be treated. You need not be one of these people; if you are genuinely happy with your girlfriend, you can show her this without expecting something in return. Showing appreciation to your girlfriend tells her that you are happy and is essentially a thanks for being a part of your life and the relationship.
Acknowledge her sacrifices. For example, if she always gets dinner for you two on the way home from work or is the one to walk the dog, tell her outright that you appreciate the things she does for you.
Make her dinner and rent a relaxing movie when you know she needs a break from life. Work, school and the general stress of life can be taxing, and she will love knowing you appreciate her enough to give her a chance to relax.
Make time for your girlfriend. Never allowing yourself to be too busy shows her that you appreciate both her and her time. Do something with her that she has wanted to do for some time, such as taking her to a new restaurant she's had her eye on.
Listen to her whenever she speaks. A person who tunes out seems like he doesn't really care, so listening to every word shows that you appreciate her and care about her needs.
Remind her of little things she might forget. This coincides with the previous step, proving that you pay attention to what she has to say and showing that you are caring enough to alleviate her stress by not letting her forget something important.