Once you've cut fresh fruit, it's only a matter of time before it starts turning brown. Slow this process by placing the fruit in a container and filling it with cold water for storage.
As food blog The Kitchn points out, this trick can be used for a variety of fruits and has the added benefit of not tampering with the taste of food like lemon juice would:
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.
You won't want to completely submerge your food in water, lest you soak your guacamole through. However, a bit of water can keep the flavour in and make your fruit last longer in storage.