Some quite good points from Physicist/Feminist:

The “What” vs. the “So What”: Doumont stressed the idea of getting across your message.  He differentiated the message from the information.  The information can be thought of as the “what”.  The message is therefore the “so what”.  One of the most useful things he said was to “maximize what the audience gets out of the presentation, not the information you put it.”  I think it can be really tempting to put as much information into your presentation as possible, but it is more effective to parse out unnecessary information and concentrate on the “so what”, the motivation for your work.  Your talks should always have a message.

You should read the full article, it has lots of great tips.

The point of maximizing for output not for input is key. My current method to try to get to that is start with a much larger slide deck, dumping in everything I find interesting about the topic. Then I start aggressively editing. Giving yourself time to edit is the key, because everyone’s first draft leaves a lot to be desired.