Avocados contain molecules called phenols which, when exposed to oxygen in the air, turn into molecules called quinones. These quinones then join together and form the pigment melanin (yup, the same one you have in your skin) that will make your vibrant avocado-coloured dip quickly turn brown. Avocados actually contain an enzyme that speeds this whole process up.
But if you stop the surface of your guacamole being exposed to the air, you can stop this happening. An easy way to do this is to cover it in a layer of water. It'll look gross, but the water won't mix with the guac, and it will stop your lovely dip browning. When you want to eat some more, just tip off the water and give the guacamole a mix.
If you have cling film you can use that instead. Make sue you press it against the guacamole so there's no dip left exposed to the air.