This is one of those presentation tips that sound much easier than they are – especially when you need to relate ideas or show comparisons among elements. Not impossible, though :).
Instead of trying to pack more into a slide, try to take away – reduce (excise) until you can communicate what you want to without having additional clutter on the slide. Can you keep your slides very simple, very clear, free of bullets, and constrained to one idea per slide?
Here are some examples, first from Steve Jobs. The contents of the table below are from the book “The Presentation Secrets of Steve Jobs.”
Note how little information is on each slide, and how each slide communicates only one idea:
You can see here a 60-second summary of his presentation, and take a look at the original slides (note that he goes with 4 main points, not 3, but you get the idea of keeping the number of main points small and manageable):
The slide below presents two groups of variables, but it sticks to one idea per slide (the correlation between Reading blogs and Learning.)