Our specific responsibility in the pursuit of holiness as seen in 2 Corinthians 3:18, then, is to behold the glory of the Lord as it is displayed in the gospel. The gospel is the "mirror" through which we now behold His beauty. One day we shall see Christ, not as in a mirror, but face to face. Then, "we shall be like Him, for we shall see Him as He is" (I John 3:2). Until then we behold Him in the gospel. Therefore, we must "preach the gospel to ourselves every day."
To behold the glory of Christ in the gospel is a discipline. It is a habit we must develop by practice as we learn to preach the gospel to ourselves. As I have repeatedly said, although sanctification is the work of the Holy Spirit, it is a work in which He involves us... we will be looking at other disciplines that we must practice in the pursuit of holiness. But none is more important than the discipline of beholding the glory of Christ in the "mirror" of the gospel.
Jerry Bridges, The Discipline of Grace