I have a couple of ways I retard the browning process. My favourite method is simply covering the avocado or guacamole with a piece of plastic wrap. Press it firmly against the guacamole and press out any air bubbles. Chill for up to 2 days. The other method is to cover it with neutral taste oil then cover it in plastic wrap. If the guacamole is completely covered with the oil it stops the oxidation of the fruit and preserves it for a short while. Pour the oil off when ready to serve. Add a little extra lime juice to remove any residual oiliness.