MM mantra 19

yat kṛṣṇa-praṇipāta-dhūli-dhavalaṁ tad varṣma tad vai śiras
te netre tamasojjhite su-rucire yābhyāṁ harir dṛśyate
sā buddhir vimalendu-śaṅkha-dhavalā yā mādhava-dhyāyinī
sā jihvāmṛta-varṣiṇī prati-padaṁ yā stauti nārāyaṇam
Synonyms: 
yat — which; kṛṣṇa — to Lord Kṛṣṇa; praṇipāta — from bowing down; dhūli — with dust; dhavalam — whitened; tat — that; varṣma — topmost; tat — that; vai — indeed; śiraḥ — head; te — those two; netre — eyes; tamasā — by darkness; ujjhite — abandoned; su — very; rucire — attractive; yābhyām — by which; hariḥ — Lord Hari; dṛśyate — is seen; — that; buddhiḥ — intelligence; vimalā — spotless; indu — like the moon; śaṅkha — or a conchshell; dhavalā — shining white; — which; mādhava-dhyāyanī — meditating on Lord Mādhava; — that; jihvā — tongue; amṛta — nectar; varṣiṇī — raining down; prati-padam — at every step; — which; stauti — praises; nārāyaṇam — Lord Nārāyaṇa.
Translation: 
That head is the loftiest which is white with dust from bowing down to Lord Kṛṣṇa. Those eyes are the most beautiful which darkness has abandoned after they have seen Lord Hari. That intelligence is spotless-like the white glow of the moon or a conchshellwhich concentrates on Lord Mādhava. And that tongue rains down nectar which constantly glorifies Lord Nārāyaṇa.
Purport: 

Devotional service to Lord Kṛṣṇa gradually spiritualizes and beautifies all one's senses. Ordinary people may not see how a Vaiṣṇava is being transformed, for only a devotee can appreciate the actual beauty of other devotees. Therefore Śrīla Rūpa Gosvāmī cautions us in his Upadeśāmṛta (5) against judging a devotee superficially: "One should overlook a devotee's being born in a low family, having a body with a bad complexion, a deformed body, or a diseased or infirm body. According to ordinary vision, such imperfections may seem prominent in the body of a pure devotee, but despite such seeming defects, the body of a pure devotee cannot be polluted. It is exactly like the waters of the Ganges, which during the rainy season are sometimes full of bubbles, foam, and mud [but which remain pure and thus able to purify one who bathes in them]."

Often, however, the transforming power of devotional service is dramatic. Śrīla Prabhupāda would sometimes recall how when he first met many of his future disciples, they were dirty, morose hippies. But as they took to Kṛṣṇa consciousness, Prabhupāda said, they became like bright-faced angels from Vaikuṇṭha.

In the course of the Lord's pastimes, the Lord will sometimes personally cause dramatic changes in His devotees' bodies. As Lord Kṛṣṇa entered Mathurā He met a young hunchback girl who anointed Him with sandalwood pulp that had been meant for King Kaṁsa, and in return for her service the Lord straightened her body and changed her into a beautiful girl. Similarly, Lord Caitanya instantly cured the leper Vāsudeva. The ultimate bodily transformation takes place when a devotee gains his svarūpa, his spiritual body, and enters the spiritual world to worship the Lord in Vaikuṇṭha.

A devotee becomes beautiful by humbling himself in the dust as he offers obeisances to the Lord. By contrast, a proud person who is trying to impress the opposite sex with his or her so-called beauty will avoid bowing in the dust. But King Kulaśekhara recommends it as a kind of beauty treatment. True beauty means that which is pleasing to Lord Kṛṣṇa.

The devotees' eyes become beautiful by seeing the most beautiful form of Kṛṣṇa. A reflection of Kṛṣṇa's radiance shines in the eyes of devotional mystics. Those who saw His Divine Grace Śrīla Prabhupāda saw this radiance in his eyes.

The spotless intelligence referred to here is one that is cleansed of all doubts and filled with pure faith in the Lord. One who has attained such clear buddhi, spiritual intelligence, is peaceful and is able to solve all problems, both his own and others'. Therefore the devotees' intelligence is likened to the moon, whose cool, soothing beauty can be seen and appreciated by everyone in the world. Similarly, the tongue of one who glorifies the Lord is said to shower down a rain of nectar, which, like the moonshine, is available to all without distinction.