One of the struggles many preachers face is selecting passages to preach. It is often easier to plan a year’s preaching schedule than to select a passage for a stand-alone message. I don’t have a definitive solution, but perhaps a helpful suggestion.
When selecting a passage there are two extremes on the scale. At one end there are passages that seem so easy and so “preachable.” Perhaps you’ve preached them before, or they are very familiar and seem to yield a message and structure just by looking at them. At the other end of the scale there are those killer passages that make you wonder how they could ever be preached. These may be tempting if you like to take on the great challenges and prove yourself. I suggest you keep a list of those that sit in between these extremes.
As you are reading through the Bible, or researching other messages, keep a log of possible preaching passages. These are passages that would take some work, but you are attracted to. Passages that seem stimulating as you read them, and would be stretching if you had to preach them. I would hope we agree that every passage is worth studying and preaching. However, we should not overlook the extra motivation that we have for some.
When you have the chance to preach a stand-alone message, take the opportunity to study where your motivation points you. But let that be a motivation for study, driven by a desire to know God through His Word, rather than a desire for an easy preaching option. Your attraction to the passage and motivation for its study may prove to be an intangible ingredient that adds zest to the message, stirring in your listeners the same motivation and desire for God.
Biblical Preaching 
Peter, great suggestion. I usually plan sermon series each year but there are always those times where you need a fill in lesson. Thanks.