Yehuda ben Teima said: Be brazen as a leopard, light as an eagle, swift as a deer, and strong as a lion, to fulfill the will of your Father in heaven.
Pirkei Avot 5:20
Although brazenness is a negative trait, one should "borrow" it when necessary, such as to do a mitzvah despite the mockery of cynics. But one should not be become brazenfaced--one's brazenness should not be apparent on one's face.
When a person's brazenness is fueled solely by the desire "to fulfill the will of his Father in Heaven"--not by his ego--it bears no negative consequence.
If your evil inclination tells you, "How can you pray to G-d? Are you not embarrassed of your petty and depraved deeds?!"--be brazen as a leopard and respond:
"True, I am not worthy. But I trust in G-d's goodness and that He will not turn me away empty-handed."
R. Naftali of Ropshitz would relate, that when he was a child and attempted to rise early in the morning to serve G-d, his evil inclination would tell him, "Why do you need to get up so early? It's still dark outside!"
The young Naftali would respond boldly: "You have already risen to do your job--but you're telling me not to rise for my job?"
Holtzberg Memorial Edition