You may not be a designer, but your presentations also don't need to scream I'm visually incompetent! GitHub developer Zach Holman -- self-proclaimed as "expressly not a designer" -- nonetheless made this beautiful presentation, and after wowing a whole lot of people, put together this slide design guide to help other non-designers get started down the road to attractive presentations.
The post is full of helpful, easy-to-apply advice, ranging from choosing your colour palette, working with text, pacing your slides, and so on. For example:
Colour is the very first thing people will notice. It should also be the very first thing you think about.
Head to a colour site like Colour Lovers and find a palette you like. Pick colours with a lot of contrast -- it gives you a lot of flexibility with text, backgrounds, and objects. In my GitHub talk, I have around four colours I use frequently, and 8-10 total (lighter green to augment a darker green, lighter blue for my dark blue, and so on).
Hit up the post for the full rundown, and while you're brushing up on your presentation skills, this guide to diagrams might also be worth a gander.
Slide Design for Developers [Zach Holman]