I've tried so many things, but there is only one thing that seems to work. Spread your leftover guacamole evenly in Tupperware, cover with a tiny layer of water (just enough to cover the guac), and seal with lid or tight plastic wrap. The water keeps the guacamole green and fresh. When you are ready to eat it again, strain out the water and dig in! I will say...this is not a perfect solution. Your guacamole can get watered down with a little flavor dilution, depending on how thick it is. I ended up adding some extra salt on the leftover guac because of this.