There are a few ways to stop the avocado in guacamole from turning brown. If you want to make it ahead of time, add a little extra lime juice to the mix. When storing it, add an avocado pit into the bowl with the guacamole - it really helps! Cover the guacamole with a layer of cling film that touches the surface directly. You can also add a layer of olive oil over the top (it won't soak in) and drain before serving.