While lack of PowerPoint skills is hardly the major problem, here's a few rules to go by:
Don't compare objects with red and green. Lots of people have red-green color blindness.
Use sharp contrasts. Colors on the projection screen may look quite different than on your computer monitor.
Use a simple font that's easy to read, Arial bold is a good example. No fancy script type fonts
No logos or templates. A blank white sheet with black font is normally perfect. In a very large auditorium it can help to reverse things and have a very light font against a dark slide.
Write big. 24 Arial bold for text, 36 Arial bold for titles. 18 and 32 is passable. If for some reason (???) you need to put something up with a smaller font, point to that part of the slide and tell the audience they don't need to read it.
Try not to go to the edge of the slide, you never know if the projection will show the complete slide.