Give her gifts homemade or not, but make it thoughtful. Don't wait for holidays either. This will let her know that she is on your mind and make her feel more important to you.
Your friends are important, but not as important as she is. If you have the choice of friends over her there are times that you need to choose her. If you don't she'll feel like she isn't important to you. Be careful around your friends that are girls too. She may trust you but do it too much and she'll start to loose faith.
Have some playful times. Play wrestle each other, play video games together, or other things that you both enjoy. This will make her feel happy and special to you.
Every girl has the look, when she is upset about something and really needs to talk to you, but doesn't know how. Figure out what her look is, ask her what is wrong and try to help her through it, even if she says she doesn't want it or doesn't want to talk about it
If you see her crying whether it's your fault or not you need to be there for her and comfort her. If you walk away from her it will make her feel worse and make her think that you don't care about how she feels