The best way to store guacamole if you aren't serving it right away or have leftovers is by quickly placing the Guacamole into a Tupperware or plastic container. Then, cover it with plastic wrap by pressing the wrap down so no air can touch the Guacamole. Lastly, place the lid air-tight on the container. Avocados oxidize when sliced open, like a peeled potato, which gives it a nasty-looking brown color. By keeping the air away, you can prevent this from happening. The Guacamole will be good in the fridge for about 3-4 days, but it's best served right after making it.