Build a presentation up, don't cut it down!
One of the most common mistakes is to start with a mountain of data and ask, "What can I cut?" Everything seems indispensable. As a result, scientists typically include far too many slides and rush presentations.
A more effective strategy is to identify the core message and "build up" from that message; adding detail in a manner consistent with audience knowledge and available speaking time. Add only detail that supports the core message. Start with a one minute version for a relatively unspecialized audience and ask, "What would I include if I had five minutes?" Repeat the process until you can fill the time that you have available.
The above slide shows this as building the presentation "up" from a single slide rather than "cutting" down from many slides.
More guidance on how to develop that core message in future tips.