One or two facts per slide is best. The minimum font size for a transparency or slide is 18 pt. Use standard fonts, in plain, bold or italic. Underscore is old fashioned. Avoid irrelevant slides.
Sport scientists are notorious for showing slides of athletes unrelated to their talks.
Check for spelling errors. List any collaborators and funding agencies here or on the next slide, and call attention to them. A relevant joke slide is OK, especially at the end or beginning of the talk. Count on each slide taking 1-2 min to explain. Don't back track if you want to show the same slide twice.