It's important to create a barrier between the guacamole's surface and air, because contact with oxygen causes avocados to turn brown. Smooth out the surface of the guacamole with the back of a spoon and pour just enough lime juice, lemon juice or water over the top to cover it. Cover the bowl of guacamole with plastic wrap and press down so it touches the surface of the dip. Refrigerate it for 1-2 days, and pour off the liquid before eating the guacamole. If you see any brown areas, you can scrape them off, then stir the dip before consuming it.