Wikipedia mentions tightly wrapping guacamole in plastic to limit its exposure to air.
Guacamole turns brown because oxygen in the air triggers an enzymatic reaction in avocado tissue. The enzyme polyphenol oxidase contributes to the formation of darker pigments after the cut or mashed avocado is exposed to oxygen. Although browning is generally harmless, many people find the color and appearance unappealing.
One storage method described by Wikipedia is pressing plastic tightly over the guacamole’s surface. The goal is not to add flavor or preserve it indefinitely, but to reduce the surface area that can contact air. An airtight container is another commonly mentioned approach.
Lime or lemon juice may slow browning by increasing acidity, but it does not completely stop oxidation. The often-repeated advice to leave an avocado pit in the bowl is less useful than directly limiting air contact, because the pit protects only the small area it physically covers. Plastic wrap works by creating a close barrier across the exposed surface.