According to a Chipotle representative, the best way to store guacamole for optimal, fresh leftovers is by using a triple-protection approach, plus refrigeration.
Specifically, this entails covering the guacamole tightly with plastic wrap, sealing it in a container with a lid, and then dunking it into an ice bath to preserve total freshness.
This approach prevents oxidation and maximizes freshness. The plastic wrap and well-fitted lid are helpful, but the chilly ice bath is what makes it completely airtight.
Keep in mind that this tip will not prevent guacamole from rotting. Rather, it helps to prevent browning due to the natural process of oxidation, which can happen quite quickly.