What is "Livyas Chen" for your head?
Rashi: A connection of grace to your head. I.e. Torah and Musar should accompany your head.
R. Yonah (here and 4:9): Musar Avicha and Toras Imecha. Put your mind to contemplate them! The Mashal is, via accepting Musar and good conduct, Chachmah will be glorious and beautiful, if he will reach the level of Chachmah and Da'as. His words will be accepted on the hearts of those who hear him. The head is a Mashal for Chachmah; there is Chachmah and the powers of Sechel. It says "v'Da'as l'Nafshshecha Yin'am" (2:10) - Chachmah will make your Da'as and Nefesh pleasant.
Malbim: It is an ornament connected to your head, on the outside. Chachmah does not come from the Nefesh internally, like Binah; rather, it is received from outside. The head represents thought and Sechel, which dwells in the head.
Ma'alos ha'Torah (DH Yafutzu): Do not say that just like wine is sometimes bad for the head and body, also Divrei Torah - "Livyas Chen Hen l'Roshecha"!
Ma'alos ha'Torah (DH ba'Derech, citing Eruvin 54a #1): One who walks on the way without an escort should learn Torah - "Ki Livyas Chen Hem" (its merit will guard him).
Eruvin 54a #2: It is a cure for the head [and the entire body - "ul'Chol Besaro Marpei" (4:22)]. If one's head hurts, he should engage in Torah - (Toras Chayim - one may not use Divrei Torah as a charm to heal, but one may learn Lishmah, intending that the reward help to heal.)
Why does it say "va'Anakim l'Gargerosecha"?
Rashi: They are like gold ornaments (necklaces) for your neck. It says Gargerosecha (plural) because the windpipe consists of many rings.
R. Yonah: The neck is a Mashal for speech, for speech passes through it. Also speak [Divrei Torah] - they are a necklace for your lips! Ba'alei Musar said, how nice is Chachmah, which beautifies deed, and how nice is deed, which beautifies Chachmah.
Malbim: It is an ornament connected to your neck, which represents speech. These Musarim should be in your thoughts and speech.
