Systematic planning is the first concern. This can include consecutive exposition of the Scripture as in book studies. It may also include a plan that emphasizes congregational needs such as special seasons of prayer, Bible reading, stewardship, etc. In a small mission church the Pastor may often emphasize outreach. When I was a stated supply for a church for 2 years, and they were in the process of calling a new pastor, I preached a series from I and II Timothy. One must be careful not to make any emphasis into a boring constant.  And then there are special holidays. I always preached continuing series on Books of the Bible. When special holidays came I would suspend the series at Christmas, Easter, etc. and resume right after. It never seemed to cause any problem. In fact, I preached mini-series during the Christmas and Easter seasons during the interruption.

The big issue here is a plan because one needs to keep track in order to be sure that he is not avoiding specific parts of Scripture, or repeating the same ones over and over. We are commanded to preach the whole counsel of God and ALL scripture is inspired of God and profitable.