The following best practices are useful when creating slides for presentations at PyCon. They're gathered from conversations with attendees and speakers alike, and they can help provide a nice experience for your audience.
The projectors currently used at PyCon are set to a 4:3 aspect ratio, so configuring your presentation software with that setting (often the default) will produce the proper slide size.
The Zen of Python says "Readability counts." Since you're presenting on Python, including code examples at some point in your presentation is likely. However, including code can be different than any of the other text-based slides you'll write.
Generally, fewer words on slides are better, and the same goes for lines of code. It's better to optimize for readability over correctness by including small snippets of code. Consider sharing a link to complete working example code that you've uploaded on the web rather than including a long function.
While you're building your slides, take a few steps away from your screen to emulate being toward the back of your presentation to see if you can actually read it.
We commonly hear that code slides are hard to read, thanks to a number of factors -- some a presenter can control, some they can't. One way to take control and provide the most readable slides is by choosing to present code with dark text on a light background.
While other color schemes work well in other environments, the combination of our room layouts, lighting, and projectors make for a situation where not all colors work equally well.
Syntax highlighting is helpful while you're writing code, but it doesn't usually add value to a short snippet of code on a slide. The readability of dark text on a light background goes further than attempting to match the code editors we often use.
For the benefit of people in the back of the room, consider how much content you're putting on each slide. While everyone can see the top of your slides, not everyone can see the bottom.
Bryan Veloso gave a lightning talk at PyCon 2012 titled Recruiting Designers for your Open Source Project with "for the benefit of the people in the back, I refrained from putting anything important here" taking up the bottom portion to shine light on the fact that people might be missing out on those last few lines of a slide filled with content.