Show her affection. Girls want guys to show them affection because it tells her that you care about her, and that you're willing to show that love in a public way. It doesn't always have to be public (PDA), but get used to the idea, because she wants other people to see how much you love her, so don't be afraid to kiss her or hold her hand in public.
Tell her she's beautiful and always make eye contact. Eye contact shows you mean what you say. When you look away, it will make her question your authenticity, so make eye contact at all times. You have to show her that you mean what you said. If you're really sincere when you tell her how beautiful she is to you, you're going to go a long way.
Also, tell her that she drives you crazy, and that she's the prettiest girl you've ever laid eyes on. Tell her that you've never met anyone like her.
Compliment her on her looks, but also on her accomplishments, talents and abilities. If she's a great poker player, tell her she's smart. If she's a good listener, tell her she's great with other people.