Related: write down what you're seeing (or rather, what you _think_ you're seeing), and so with pen and paper, not the keyboard. You can type way faster than you can write, and the slowness of writing makes you think harder about what you think you know. Often you do the know the answer, you just have to tease it out. Or there are gaps in your knowledge that you hadn't clocked. After all, an assumption is something you don't realise you've made.
This also works well in conjunction with debug tooling -- the tooling gives you the raw information, writing down that information helps join the dots.