What is the most important thing to do in our preaching?

What would you say is the single most important thing in preaching—either as the person preaching or as someone who listens? I guess many people would suggest clarity of delivery, or humour, or connecting with the congregation, or being based in Scripture. All of these are of great importance, though of course all are open to a range of interpretations.

As I continue to preach myself, and sit and listen to the sermons of others, I keep coming back to something I first learned more than 25 years ago, and feel I keep on needing to learn:

Focus more on what God has done, 
and less on what we ‘ought’ to do.

Why is this important? I think for several reasons.