A pastor can make wise use of commentaries and other scholarly helps in sermon preparation by using such tools as conversation partners, not as masters.
How? By spending your earliest and best time with the text itself. Meditate on the text. Pray through the text. Exegete the text thoroughly. Outline the text. Ask questions of the text and answer them all yourself before you consult any commentaries.
A pastor can make unwise use of commentaries and other resources in sermon preparation if he allows them—rather than the text itself—to set his sermon’s agenda.