Some fantastic advice here from the Presentation Zen site (which is always worth looking at). Many scientific presentations would be greatly enlivened if presenters took more effort to turn a collection of facts and observations into a story. Tip #4 is something that I frequently mention to students in our lab:
(4) Have a clear theme.
What is your key message? What is it you REALLY want people to remember? What action do you want them to take? Details are important. Data and evidence and logical flow are important. But we must not lose sight of what is really important and what is not. Often, talks take people down a path of great detail and loads of information, most of which is completely forgotten (if it was ever understood in the first place) after the talk is finished. The more details that you include and the more complex your talk, the more you must be very clear on what it is you want your audience to hear, understand, and remember. If the audience only remembers one thing, what should it be? Write it down and stick it on the wall so it's never out of your sight.
Sometimes students seem almost surprised by the notion that the audience should be expected to remember something from their talk.