Here is a lecture by Fr. Valery Lukianov (who is one of the most senior priests in ROCOR, and known to be an exceptionally good preacher) on how to prepare a good sermon:
You can listen to Fr. Thomas Hopko discuss sermon preparation and delivery here.
Here also is some wisdom from a Protestant minister -- Fred Craddock -- who is considered to be one of the greatest Protestant preachers alive today. Not everything he says would be applicable to Orthodox preaching, but there is much that does:
Purpose of a Sermon:
What to avoid when preaching:
Storytelling in Sermons:
On Using Humor or Emotions in Sermons:
You may also want to look at the Preacher's Institute web page (an Orthodox Website):