I usually don’t draw diagrams, but my mind works better with what I usually call “tiny examples”: working code unrelated to the main codebase. Most of the time I have pen and paper with me, but I never do anything fancy with it. The advantage of that is most of the times, when I’m done, I need only to copy the example to the actual codebase with small changes.

This process helps me in so many different ways, and seems so productive, but I’ve never found anybody that uses such an approach. I would love to discuss it further with other people so that I can improve it.

One of my most successful development experiences came from starting not with code, but with a keynote/powerpoint slide deck. It made me think through exactly what it was I was trying to achieve, what it would involve, and how it would work. When I realized something or other didn't work well, changing approaches only required editing a few high-level diagrams. Computer diagrams can be cumbersome compared to pen and paper, but have the great advantage of being easy to change once you've spent the time to draw them. Implementing the final solution was much easier when I could look back on how it was supposed to work. And at the end, I already had slides ready to explain everything...