One of the best ways I have found not to get distracted while in a task, with other more interesting or something that you do not want to forget, is NOT to use your device to do it or even add to your TODO, or note it down but to use an actual notebook (may be a small memo note) to quickly write out and continue with your current task/more.

When you are done, visit and parse your sidenotes.

Of course, the are the usual No Notifications, timers, etc.