> Does anyone have pointers on how make your slides look
> beautiful without being a trained designer?
By keeping it simple. Pick one good font. As little text as possible. No templates[1]. Solid image material (sufficient resolution, used sparingly, do not overcrowd the slide with superfluous visuals). Without much work, you've outdone 90% of your competition.[1] By that I mean: No fancy pre-made visual templates. For the sake of consistency, using the templating mechanism of your chosen tool (Keynote, PowerPoint, Beamer[2]) is highly recommended. But you'll have to create your own templates.
[2] Don't use Beamer.