Keller recently wrote an article in Christianity Today about this subject of gospel humility and self-forgetfulness. It's a great summary of the sermon he gave several years ago. Here's a couple of paragraphs:
"Christian humility is not thinking less of yourself; it is thinking of yourself less, as C. S. Lewis so memorably said. It is to be no longer always noticing yourself and how you are doing and how you are being treated. It is 'blessed self-forgetfulness.'Read the whole thing
Humility is a byproduct of belief in the gospel of Christ. In the gospel, we have a confidence not based in our performance but in the love of God in Christ (Rom. 3:22-24). This frees us from having to always be looking at ourselves. Our sin was so great, nothing less than the death of Jesus could save us. He had to die for us. But his love for us was so great, Jesus was glad to die for us."