Fruits tend to turn brown shortly after being cut. But you can slow down this process by placing the fruit in a container with cold water for storage.

Pointed out by The Kitchn, the trick can be used for a variety of fruits.
This method, on the other hand, works perfectly every time and doesn't involve plastic or extra lime juice. I like a precise balance of lime to avocado in my guac, and I don't want to drench the guacamole in juice to keep it fresh. This method is easy and cheap, and you can use it with any kind of guacamole. All you do is cover the guacamole with a thin layer of water.
[The Kitchn]