I remember how skeptical I was looking at some presenters traveling around to multiple conferences with "the same" presentation a decade or so ago. Having been a course instructor for years and being a presenter for even longer, it looks completely different now. It's not that the topics you cover under the same looking umbrella can be quite different, you also find much better ways to express whatever you want to tell your audience as you experience feedback.
Of course the best would be to present your story crystal clear from the start, but despite being an enthusiastic follower of Garr Reynolds and Nancy Duarte, you'll undoubtedly need lots of time anyway to take a relaxed look on your story and distill to the level needed to form a great presentation. I've actually found it quite hard to refine my slides without actually showing/presenting them to an audience. The faces, questions, smiles and sometimes plain staring expressions you get tell you how you'd done and you can derive ways of how can you improve.
Two interesting examples are my slides on Drupal 7 and localize.drupal.org.
