The stages of sermon preparation are not a hard and fast series of steps. It is possible to have a useful thought for the introduction, conclusion, illustrations, and so on, very early in the process. Yet these are all stage 7 elements – message details. So even though it is possible to have thoughts at any time, it is usually better to note them and leave them until later. This is especially important in stage 2 – passage study. A commitment to expository preaching requires that we keep stage 2 unpolluted by stages 5-7.
1. As you are studying your passage you are not looking for a sermon. If you collapse stage 6 – sermon shape, into stage 2 – passage study, you will undermine the whole process. It is critical to study the passage first, to understand it, rather than to form it into a sermon.
2. If your mind creeps ahead, make a note and get back to stage 2. We’re all tempted to see our points as we study. Write them down and put them aside. That is not yet. We easily look for our sermon structure, will there be two points, or three? Inductive or deductive? Don’t. Write down any thoughts and then put that aside.
3. Be clear on your goal in studying a passage. What is the goal of studying the passage? It is not to find the sermon. It is not to determine the points of the sermon. It is not to utilize our Greek or Hebrew until we feel we have fulfilled some sense of duty. It is not to parse verbs endlessly, or do word study after word study. The goal of studying the passage is to find, with some degree of confidence, the passage idea. The goal of stage 2 is stage 3 (and part of stage 4). The goal of studying the passage is to know what the author’s idea was, and why he wrote it. Seems obvious, but we easily forget. In fact, many of us have never been told that. I don’t recall my seminary profs training me to exegete a passage so that I grasp the author’s main idea. But that is the goal. All the Bible study skills we have are there to work towards that.
Determine the main idea of the passage, with as much confidence as you can achieve in the time you have. Then you are ready to start considering the purpose of your sermon, your sermon idea and your sermon outline. Do these things too soon and you may abort your Bible study.
Biblical Preaching 
While I certainly agree with the necessity of keep stage 2 “pure”, I think there is another way that will help us to do that. You mentioned under number 3 the necessity of studying the passage “to know what the author’s idea was, and why he wrote it.” We must learn to do this at all times, not just in sermon preparation but also in our devotional reading and as we read or listen to what others say about a passage. It must become habitual with us. How often I have asked a question in a Bible study discussion group about a text that could be answered from the text but the answers I would get, although not wrong, had nothing to do with the text. People do that because they don’t think habitually about what the author is saying.
Enjoyed this post as always. Keep up the great work.