Put space between things
Related to the "Do less" rule, but it's a way of managing your schedule so that you always have time to complete each task. Don't schedule things close together - instead, leave room between things on your schedule. That gives you a more relaxed schedule, and leaves space in case one task takes longer than you planned.
This is very hard one for me and might you too. We are so addicted to scare free time with no companies and no plans. Fill the empty slot of your time like playing "Tetris"....... It's totally OK there is no game over with having empty space!