This is a pretty good piece from Tim Keller in Christianity Today, even without the frequent Lewis quotes. An excerpt:
We are on slippery ground because humility cannot be attained directly. Once we become aware of the poison of pride, we begin to notice it all around us. We hear it in the sarcastic, snarky voices in newspaper columns and weblogs. We see it in civic, cultural, and business leaders who never admit weakness or failure. We see it in our neighbors and some friends with their jealousy, self-pity, and boasting.
And so we vow not to talk or act like that. If we then notice "a humble turn of mind" in ourselves, we immediately become smug—but that is pride in our humility. If we catch ourselves doing that we will be particularly impressed with how nuanced and subtle we have become. Humility is so shy. If you begin talking about it, it leaves. To even ask the question, "Am I humble?" is to not be so. Examining your own heart, even for pride, often leads to being proud about your diligence and circumspection.