There are many ways and methods floating around about how best to keep avocado from turning brown. Depending on what I am doing with the avocado, I have a few different tried-and-true methods.

In the case of guacamole, whether I am making it ahead, or saving the remainder for a second service, I keep it from turning brown in the exact same way. Cover it directly with saran wrap, leaving no space for air. You don't want to simply cover the top of your bowl, but instead, press the wrap down directly touching the top of your guacamole.  This same method applies if you are storing your guac in a Tupperware or other vessel with a lid. You want to make sure that you layer saran wrap over the guac and THEN put the lid on. The key here, is to make sure that there is no air touching any part of your guacamole. If there is no air, there is no brown.