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.]