Martin Luther [update: apparently a paraphrase of/commentary on his perspective]: "Because heresies threatened the living apostolic message, it had to be recorded in a book to protect it from falsification. Preaching reverses this process of conservation again, allowing the Scriptures of the past to become the tidings of the present . . . The Gospel has been committed to lifeless paper; fresh words can transform it into glad tidings again."
William Ames: "Next to the evidence of truth, and the will of God drawn out of the Scriptures, nothing makes a sermon more to pierce, than when it comes out of the inward affection of the heart without any affectation. To this purpose it is very profitable, if besides the daily practice of piety we use serious meditation and fervent prayer to work those things upon our own hearts, which we would persuade others of."
[I don't have the bibliographic information for either of these; if anyone has this, feel free to leave it in the comments.]