Listen to her, I mean really listen. Whether you think or know you are right or wrong. A good excercise that I learned about is to have the person who is mad, say why they are mad, and then the other person paraphrases or summarize him/her so they know you are listening. That way you can understand what the root of the problem is, and know you are on the same page.
For example:
GF: You hurt my feelings because you said you didn't have time to see me today.
BF: So you feel hurt because I couldn't see you today.
GF: Yes, so I felt neglected.
BF: I understand, you were hurt and felt neglected. Well what I meant was....etc, etc
You get the idea. Both of you should take turns, really listening to each other. Don't just offer her a quickie solution because the same problem will keep arising. Don't be afraid to say you are sorry when you realize you are wrong. Don't say "You are this, or you are that... Say what you did was hurtful, not you are hurtful". don't play the blame game, that doesn't solve anything.
That's all I can think of right now, but good luck! Fighting and arguing is natural, it's getting through those tough times that determines what relationships last and which don't.