I've tried several methods. A jar of water works best for me. Place a cut avocado half with the seed still in it inside a pint jar or container. Fill with water until it covers the avocado and store in the refrigerator. Use within 1-2 days. When you get ready to eat it, you may observe the very top layer of the avocado flesh seems a bit water-logged. But when eating mashed avocado on avocado toast, for example, I don't notice it.