"that you may proclaim the excellencies of him who called you out of darkness into his marvelous light" (1 Peter 2:9 ESV).
It is our supreme joy to declare how excellent is Jesus Christ, how excellent is the Father who gave him to us, how excellent is the Holy Spirit who led Christ and leads us. Preaching should be about grace: God's kindness to us based not on anything in us but wholly on his abundant goodness.
"But we have renounced disgraceful, underhanded ways. We refuse to practice cunning or to tamper with God's word, but by the open statement of the truth we would commend ourselves to everyone's conscience in the sight of God" (2 Corinthians 4:2 ESV).
Preaching should not use guilt as a motivation, should not play upon emotions to produce artificial responses, should never sugar-coat difficult truths with misdirection as is often seen on television. Instead, by the open statement of the truth, we commend ourselves to your conscience before God. If you read on in 2 Corinthians 4, you'll see that God's grace is sufficient cause to trust in plain honesty.
"...for I did not shrink from declaring to you the whole counsel of God" (Acts 20:27 ESV).
Biblical preaching should cover the whole Bible, adding nothing, subtracting nothing. Therefore, firstly, the main point of the passage must be the main point of the sermon. The pastor must not dream up his own ideas and then look about for any random Bible sentences that seem to support him. He is instructed to preach the counsel of God, not his own counsel.
Secondly, my preference is to work through the entirety of a book of the Bible. This "sequential exposition" of a whole book over the course of several meetings allows the preaching and teaching
"All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work" (2 Timothy 3:16-17 ESV).
Because the Bible, all of it, is profitable, Biblical preaching should be relevant to today's world, interesting, easy to follow, applicable to modern problems and situations, useful both in the content and in the attitude of the delivery.