Tell her you love her. Make sure you mean it. If you're not ready to go there yet, that's fine. But, be sure to switch that "I love you" for a really good "You know how special you are to me?" or a "I like you so much; I'm a really lucky guy."
Simple is best. Stick with "I love you," or "You make me so happy," or "I'm really lucky to have you." Don't be so poetic that you go overboard.
These statements make her feel good, and they make her self-confidence soar. Remember, when her self-confidence is high, you benefit. Be steady with your signs of love or affection, enough so that she doesn't forget how much you mean to her, but not so much that the power of the words gets watered down.
:-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-)