NERD ALERT: The act of oxidation is what turns our bright green friend brown. Oxidation happens in the presence of air. To keep your guac as green as possible, reduce its contact with air. You can do this by storing it in something that's taller rather than flatter. Also, add a layer of plastic wrap and press it into the top layer of the guacamole to ensure there is very little air contact. Another trick that I use is to smooth out the top layer of your guacamole with the back of a spoon and then very gently add a layer of water to the top of the guacamole. This sounds weird but it makes total sense. The water keeps any air from getting to the guacamole so no oxidation can happen. Store the guacamole in the fridge for up to two days. Pour the water off before serving.