Store leftover gucamole in an airtight container in the refrigerator for 2-3 days, though best when fresh.
*If you're not going to be serving the guacamole right away, save the onion for later. (Do not process the onion with the other ingredients). Once the guacamole is ready, transfer it into an air-tight container and scatter the chopped onion on top. Seal the container with a lid, and put it in the fridge. Mix the onion into the guacamole right before serving.