The guacamole will brown with time but you can try to slow it down by doing a few things. You can sprinkle some extra lime juice on top of the guacamole. The acid will help slow the oxidation process. Also, if you're making it ahead of time, smooth the surface of the guacamole and put a layer of plastic wrap directly on the surface. Then, wrap the whole bowl with more plastic wrap. This will keep as much oxygen away from the guacamole as possible. Some people say that putting the pit of the avocado in the guacamole will prevent browning but that's debatable.