[as my faithful readers may have come to understand, i'm working on a sermon training manual for laity. comments welcome]
It is also important to listen to people that are older and wiser, and those that have called on Christ in different historical periods--those that aren't you're contemporaries. There is an idea in contemporary evangelicalism that learning is not necessary, particularly book learning as it relates to the scriptures. It's thought that we have the Spirit who will lead us into all truth; God's revelation is immediate, why do we need then an intermediary resource, like a commentary?
It's interesting that throughout history this sentiment has often expressed by heretical groups and sectarian movements. The lack of interest in commentaries or learning from scholars and Bible teachers is often a mark of pride with a spiritual veneer. Also, it is most likely indicative of the enlightenments over-emphasis on the individual. The mainstream of the church has always believed that the individual is not independent but rather, inter-dependent, with regard to all things. The secondary act of listening in sermon preparation is to listen to the Church's voice expressed in commentaries and sermons on the relevant texts.
When reading a commentary you are able to sit at the feet of great scholars and Bible teachers. You are able to hear the text afresh and discern new ways to perceive and communicate scripture. You're able to stand on the backs of giants.
However, some might too quickly move to the commentary, this should not be done. Reading commentaries and listening to relevant sermons is a crucial act of sermon preparation but it should never precede a prayerful individual reading of the passage. It is incredibly important to familiarize yourself with the assigned passage, seeking God's leading in the interpretation of the passage, before you consult those older and wiser. In this way you are getting to know the text and thus the commentaries and sermons aren't speaking into a vacuum. Further, when you approach the commentary or sermon you bring your own learning to the discussion.
Pay attention to how the commentators divide the text (chapters and verses). What do they spend most of their time discussing? Do they offer important historical contextual points, things that make the text more lucid and challenging? Compare and contrast your reading of the text with theirs'.