For me the greatest challenge in preaching is finding the right ascesis for preparation. To paraphrase Master Yoda, it requires a good deal of 'unlearning what you have learned.'
Ever since I have been able to read, I have been conditioned to read a text as fast I can (while still comprehending it, more or less) and to think of something clever to say or write about it right away. The quicker and the more clever the better. This is the relationship to a text that going to school has taught me, from the first grade all the way through theological studies.
The trouble is, you just can't go about preparing to preach on the Sacred Scriptures in this way. To skim the text and go with the first clever thing you think of is a serious error, and one that has caught me many times in the hundred or so weekends I've spent so far in the clerical state. Many times I have gone to an ambo to proclaim the Gospel and only at that moment realized how I ought to have reflected and prepared to preach on it. I realize that this is partly because this is when the printed word becomes the Word of God most eminently, i.e. when it is proclaimed in the assembly, but it's also because the Scripture simply has to be heard.
Through all of this I have learned that I need to employ all kinds of practical strategies to make myself slow down in the course of my weekly preparation. I need to read the Sunday Scriptures out loud, for one thing. I need to refuse to have an idea the first time. Ideas make you miss things, as you begin to conform the text to the thing you want to say. I need to play with several ideas and refuse to choose one right away. Finally, I need to keep reading the text as I prepare, to make sure it's not drifting off into my idea. For we are not meant to preach ourselves, but Jesus Christ as Lord. (2 Corinthians 4:5)