There are 2 ways to prevent it from browning: 1. Transfer guacamole to an air-tight container, place a plastic wrap directly over the surface, pushing out the air. Then close with the lid and refrigerate. 2. Transfer to an air-tight container, press down, pour a little bit of water to cover (it will not absorb the liquid). Then cover with the lid and refrigerate. To serve, pour off the liquid, and stir. You also might want to add a spritz of lime juice to freshen it up.