Modesty:

1. One should have a strong sense of humility before God. One should feel ashamed to do anything which would not be in accordance with the will of God, and certainly ashamed of actually sinning, God forbid. Having a sense of humility is the foundation for developing genuine fear of Heaven. A person who has no shame in this world will be put to shame in the World to Come. This is the worst of all punishments. The pain of the feeling of shame in the World to Come is even worse than the bitterest suffering in Gehinnom. Even the Tzaddikim will feel embarrassed before other Tzaddikim who reached higher levels than they themselves. Our Sages said, `Alas for that embarrassment, alas for that shame!' (Bava Bathra 75). They were talking about the shame the Tzaddikim will feel. How much more will sinners be ashamed and confounded. In this world it is impossible to form any conception of the bitterness and suffering which the shame and embarrassment of the World to Come will cause. But someone who develops a true sense of shame in this world acquires the instrument with which to keep himself from sin. He will discover true prayer and joy and inner strength. He will draw close to the Tzaddikim and learn from them how to develop perfect faith (22).
2. You should be totally honest when you talk to God. Get into the habit of expressing yourself with such honesty that your heart is stirred and the words begin to pour forth with fire and passion. As you draw yourself closer to God you will see how small and insignificant you are compared with His greatness. You will be filled with feelings of humility before Him. For up till now you simply cast your sins behind your back and ignored them. But as you start to acknowledge them frankly you will be overwhelmed with a feeling of shame at having rebelled against the Master and Ruler of the Universe, the Source of all the worlds. At first this humility will not actually show on your face. This is because sin sullies the intellect and stops it from radiating on the face. Before a person repents his mind is so weakened that he can have no conception of the true gravity of sin and the greatness of the One before whom he sinned. But as he returns to God and strips himself of his folly, gaining wisdom and understanding, so his shame will become more and more visible on his face. The sense of humility is symbolized in the tefilin, which are the sign of our attachment to the Creator. The light of the tefilin is a ray of the light of God's inner countenance. When a person achieves this sense of humilty all his sins are forgiven and he becomes attached to the Tree of Life (38).
3. There are certain people who are so brash and arrogant that they feel no shame or embarrassment at all before the Tzaddik or those who are truly pious. This brashness is a sign of impure ancestry: it is very doubtful if their ancestors stood at Mount Sinai (Nedarim 20). They have impugned the holiness of the tefilin, which are rooted in the Tree of Life. Instead they have attached themselves to the Tree of Death. They will be cast out of the Garden of Eden and thrown into Hell. But a person who repents and feels ashamed of his sins will immediately be forgiven, and the wisdom and understanding of his soul will be restored. He will be able to expel the spirit of folly from within him and become worthy of the radiance of the tefilin (Ibid. 5:6).
4. Shame and embarrassment are the basic qualities which help us to return to God. The merest sin one may have committed should give one a deep sense of shame, because in reality every Jew ought to be far removed from sin. The soul of the Jew is rooted in a source so exalted that in essence he is completely detached from sin. For a Jew to sin in any way at all is totally unbecoming. Even when he wants to perform a mitzvah, a positive action, he should also feel a sense of shame and embarrassment. What right does he have to perform this mitzvah? How does he dare to enter the court of the King and perform the mitzvah when he considers the greatness of the One before whom he does it? The true significance and preciousness of each mitzvah is totally beyond our comprehension. A person has only to look at himself to see how far he is from God and how unworthy he is of performing the mitzvah. How can he stretch out his hand to take the tefilin, the very crown of the King... and simply put them on his head all of a sudden? A person who felt genuine shame would be embarrassed even to take food and put it in his mouth before God. What right does he have to this food? To acquire a sense of shame one should examine oneself in comparison with the Tzaddik. This will move him to repent and attain true humility, the humility of Moses. This humility is the root of life -- the eternal life of the World to Come (II, 72).