One main message per slide. Reduce number of elements on your slides. Whitespace, called sometimes negative space is very important part of your slide. You don't need to fill every spot on your slide. Let the slide breathe. Help the audience to identify what is important. Don't be afraid of that space on your slide. Do not be worry that your slide will look like empty one. Avoid temptation to add more and more elements to your slide.
Tips and ideas for presentation design.