"Don't wait." Waiting for something always takes more time than I thought it would, so whenever I notice myself waiting, I switch to doing something useful in the meanwhile and push the waiting task into the background. Installing the habit took a little bit of effort, but by now it's automatic.

Could you elaborate a bit on that?

I noticed that I often wait for small tasks that end up taking a lot of time. For example, I need to compile a library or finish a download and estimate that it won't take long, maybe a few minutes at most. But I find it really hard to just do something else instead of waiting. I can't just go read a book or do some Anki reps. Whenever I tried that, I either have the urge to constantly check up on the blocking task or I get caught up in the replacement (or on reddit). So I end up staring at a screen, doing nothing, just so I don't lose my mental context. At worst, I can sit for half an hour and get really frustrated with myself.