Do not call her beautiful, pretty, attractive. She is not the angle of bone nor the measured scope of her face. Neither call her beautiful on the inside, since she'll know exactly what you mean - a lack, dressed up in a compliment.
Instead, tell her how you love the way the world tilts slightly, as if the moon has come closer to see, when she smiles.
Do not say she is tall or short, fat or thin, as if we possess space and every inch of it must be paid for somehow. Instead, note the way she shines at the edges, where the bright, immortal sphere of her soul comes in contact with the human hour.
Let her know you are grateful for her body, since it brought her to you.
Describe not what she does for a living, because if we think money is living, we've got life badly wrong. Talk instead about the quality and tone of her silence as she sits in the bedroom watching rainshadows fall and tiny motes of dust fall and your whole courage fall because how can you approach this woman, this universe, breaking her silence and risking whatever she might say to shore you up or break you?
And when in the darkness you can not see her, can only dream her in the warmth of breath and love's heart beating, you must relinquish words entirely. When you know at last that she is beyond description, then you understand a woman.