I once attended a scientific talk where someone started off by stating that he had given himself an award, a clip art ribbon, for busiest title slide. Sure he was joking, but I was cringing. Sadly this is just one of far too many examples of slides that I’ve seen that would look more at home in a kid’s scrapbook than a scientific talk.
Use a clean, balanced layout
PowerPoint (or equivalent) prompts you to choose a layout from a list like this:
Notice how all the layouts are balanced, aligned, and evenly spaced. There aren’t any options for the layouts like the following:
However, given how easy it is to copy and paste images, this is often how slides end up looking. I understand that figures will come in all shapes and sizes, so it maybe a challenge to find a good layout. However, aligning and avoiding overlap should be a no-brainer.
Choose a good font
The choice of font for a scientific talk should be something nobody notices. If people do notice, then they’re getting distracted from the content. A likely reason for them to notice is an inappropriate choice. The following are classic culprits:
Choosing a font shouldn’t be that hard. Stick to a sans serif font and keep it black unless you know what you’re doing.
Ensure the background doesn’t interfere
Everyone likes to display nice photos in their talk. But too often this desire manifests itself in a background image that becomes a distraction or even a hindrance to the viewer. The image can easily camouflage the actual content on the slide if you are not careful. Personally, I forego background images except for the occasional title slide. Judicious use of partially transparent black or white regions above the image ensures the text is easily readable. For example:
To compare, consider the same image with the title simply on top of the image:
Brushing Up Science 
Comic Sans. Just don’t.
So true.