The Mayo Clinic advises against eating leftover food stored in your fridge for more than four days. How do you remember that? Put sticky notes on the container and write either the date you made the food or an expiration date.
SquawkFox, who originally suggested this idea, has an additional tip that can stop you from wasting your leftovers:
Store your leftovers at the front of the fridge at eye level so you can see them (and eat them) before they spoil.
The full post is well worth reading as she goes over several details of how to organise your fridge for maximum efficiency. She breaks down every aspect of cleaning, organising and maintaining it and has come up with a helpful infographic that you might want to print out and use for reference.