Painters first make a sketch before starting the final painting. Presentation designers should do the same. I have a big pile of old print outs that is my unlimited source of scrap paper. An important slide can take 5-10 page-filling rough sketches before turning to the PowerPoint editing screen.
I always carry one of these beautiful notebooks (affiliate link) with me to capture an idea that pops up in my head. Yes, a notebook and not an application such as Evernote on my iPhone because the idea is most of the time a sketch or a scribble. Hard to do in digital format.
The end of my most productive/creative days are always marked with a full paper trash can next to my desk.
The painting is Gauguin's night cafe, info about him and Van Gogh painting at this location here and here.