Scientists watch a lot of talks, and I’ve noticed a lot of people (including me) make the same handful of mistakes. Here are a few of my tips:

• Number your slides. Powerpoint, Keynote and Beamer all have options to add these automatically. Visible slide numbers make it easier for people to refer back to a specific slide if they have a question, especially at the end.
• Test your slides on a projector or low-resolution monitor. Computer monitor resolutions have steadily grown, but projectors technology seems stuck in 2004. This leads to a familiar trap: you make beautiful figure with graceful thin lines on your laptop, which are rendered totally invisible by the projector. Same goes for contrast, light colors like yellow are often invisible on projectors.
• Keep the text to a minimum. You want people listening to you speak, not reading your slides. Use slides for short bullet points and for showing off your figures.
• Even fewer equations. Unless you’re teaching a class, people are rarely going to be interested in following any mathematical derivations, and they’re hard to follow on a slide anyways. 1-2 equations per slide max. If people want to know more, they can always ask, which will probably lead to a more interesting discussion anyways.
• Finally, include your contact information on the final slide. It’s easy to space out at the start of a presentation and forget to jot down the presenter’s name. Make it easier for your audience by having your name and email on the last slide along with any relevant papers you want to promote.

Disclaimer: I want to be 100% clear that these tips are not a veiled reference to anyone in particular.