Smiling and giggling
A female who doesn't giggle when you're talking to her is like a cat that doesn't purr when you stroke its head and neck (or whatever the hell dogs do). How would you tell that this purr-less feline loved you for taking care of it?
Giggling is not the same as "ha ha" laughter, and it is certainly not the faux cackling that girl-haters produce in mockery. In fact, she is more likely to giggle when she's done something wrong (but not horribly so), and you give her a mock scolding look -- "[giggle, giggle] Whaaaat? [giggle, giggle] What did I doooo?" Giggling signals her vulnerability: it is designed to manipulate you into treating her more gently than you might be inclined to when she goofs up.
Young girls are also much more likely to make a drawn-out, highly inflected "Awwwww!" when they think something is cute. You will then be tempted to make that sound yourself (if briefer and less inflected) in response to how adorable they are being, but you must always suppress this reflex, as you don't want to look adorable.
(The first 17 times you watch this video, you will probably be too mesmerized by how impossibly tight and smooth her skin is to enjoy the humor.)
How to preserve this charm:
Because it's reflexive, giggling is hard to fake. But do whatever gets you into a giddy mood -- sleep deprivation helps. Maybe watch some old movies that feature your teenage crush and recall all of the silly little fantasies you had about how your first encounter would go.