While some dishes definitely taste better the next day, for the most part, leftovers aren't as good after coming out of the fridge. That's because people make these common mistakes.
According to those at Eat This, Not That, don't leave food out on the counter for more than two hours, which ups the risk of bacterial growth. Also, avoid saving them for more than three or four days.
Refrigerator temp is really important, as well as keeping those foods in airtight containers.
How dishes are reheated makes a big difference as well, so make sure you know the best practices like when to use the microwave and how to arrange the plate when nuking foods.