Most people treat leftovers as what happens when dinner goes wrong. Build them in on purpose instead and you buy yourself a free night every single week.
Two habits make it work. First, cook one deliberate extra portion whenever you are already at the stove, and put it away before you sit down to eat, not after. Second, give leftovers a real slot on the calendar. A night where the plan is leftovers is a night you do not have to think, and thinking is the part that wears people out.