If I need diced onions for three meals, I dice enough at one time for all three meals. I cut up salad fixin's for two days at a time (much longer and your lettuce could wilt).
Here I cut up lettuce and tomatoes for salad one night and Petro's the next. I stored the lettuce separately from the tomatoes in the fridge so things don't get soggy.
Find the whole "Working Ahead" series here.