Hold her hand, stare into her eyes, kiss her hand and then put it over your heart. Unless you can feel their hear beating, you arent close enough. Sit in front of a roaring fire and make out/make love. I love the way a girl looks right after shes fallen asleep with her head in my lap. Listen to classical music and cuddle in the dark or w/ blacklight. Find out their favorite cologne/perfume and wear it every time youre together. Go hiking and camp out together in the woods or on a mountain.
Keep one of her bras somewhere where you see it everyday.
Tell her about how you answered every question in math with her name.
Find a nice secluded place to lie and watch the stars. Incense/candles/oils/blacklights and music make for great cuddling/sex.
Lightly kiss their collarbone and their jawbone just below the ear, then whisper I love you.
Say I love you, only when you mean it and make sure they know you mean it.
Tell her that she doesnt have to do anything she doesnt want to.