(Hopefully other girls can relate to these and they are not just preferences)
-Do sweet things for her. It doesn't have to be much, or even all the time. Just something that shows you care.
-Take her out to meet your friends and family. If you're together, introduce her as your gf.
- Let her know she's beautiful. This one is a little tricky... don't overuse it, and don't be creepy about it.
- Don't compare her to other girls! Unless you're saying she's amazing and not like any other girl.
-If she offers to pay ask her if she is sure, and then let her know you're thankful for the gesture, and then let her pay.
- Don't be a gentleman if you're not genuine about it. If it's something you're doing just to "get the girl" then you're just wasting time.
-Respect "the flow". Okay that sounds gross, but really. Some girls do suffer from PMS, so be understanding and grateful that you're body isn't designed to have babies.
-Don't call us babies. Most of us are tougher than you think.
This last one might just be a preference, but...
when a guy takes a picture of me on his phone, I practically melt. Sure I'll probably think I look hideous, but the fact that he wanted a pic of me on his phone, that he thought I was "cute" and silly enough to be taken a photo of, well that just makes me happy.
However if your intent is to make fun of her then DON'T DO IT!