Avocado (and guacamole) turn brown when they are exposed to oxygen. To keep your guacamole form turning brown use a piece of plastic wrap to cover it. You want to put the plastic wrap right on top of the guacamole so that it is not exposed to any air. This method works best if you are preparing the gaucamole a couple of hours before you eat it. If you are storing it over night there's a good chance some air will get in and the top of the guacamole may still brown a bit. If it does, just scoop the top off and stir.