Selecting a text to preach on is half the battle in sermon preparation.
There's one advantage to preaching through a book of the Bible: you know what you're going to preach on next.
But I fill in for the senior pastor, and so I don't typically do a series.
The last two weeks I preached on 1 Thess 1:9-10 and Matt 15:21-28, respectively.
The 1 Thessalonians passage I decided on for a couple of reasons: 1) I'm teaching through 1 Thess in SS, so it was on my mind, and 2) those verses have often struck me as a neat summary of what a Christian is. My title was "Obviously Christian," and the gist of my message was that Christians should be obviously so, because they are characterized by an obvious change, an obvious service, and an obvious expectation.
A great deal of prayer goes into my selection of a sermon text. I ask the Lord to guide me to the text he wants me to preach.
The next week I leaned toward several texts before I settled on the Matt 15 one, including Dan 7:13-14; 1 Thess 2:13; 1 Tim 1:3-7; and 2 Chron 20. The Matt 15:21-28 text, on the Canaanite woman who was at first rebuffed by Jesus before her daughter was finally healed of demon possession, came to mind as one of those passages I have often thought I need to study more because I didn't understand it so well. What a rewarding passage that was to me personally as it encouraged me to persevere in prayer. I hope it was an encouragement to the congregation as well.