Leftover guacamole should be stored in an airtight container in the fridge. As guacamole sits, the surface will turn brown from oxidization. To slow down the browning (oxidization) process, you can cover the guacamole with a thin layer of water. (If you do this, make sure to pour off the water before serving!). Or, simply press a piece of plastic wrap directly onto the surface of the guacamole.