Listen to her when she talks. Remember things that are important to her, like birthdays, her likes and dislikes. A lot of guys give flowers, which are great. But my favorite present I ever ever got was a book that I had been wanting for ages. The guy was just a friend of mine, but he took the time to pick up cues from our conversations and got me the perfect gift.

Be nice to her mother/father/siblings/family.

Don't belittle her. Ask her opinions on things.
Don't talk about your ex girlfriends ad nauseum. Don't ogle other girls when you're with her. Focus on her.

Tell her she makes you feel special. Then tell her why/how.

I'm sure there's more. Those are just some.