The Secret to Storing Avocados. Once an avocado is cut, it doesn’t stay fresh — or vibrantly green — for long. Try refrigerating it in a sealed container with some chopped onion. The onion works like a preservative, and avocados will stay fresh for several days using this method. If you’re making guacamole, sprinkle chopped onion over the top and cover with tinfoil to store, then stir in the onions before serving.