Three Methods from Food52.com on How to Keep an Avocado from Browning:
The Onion Method
Roughly chop a quarter of a red onion into large chunks. Line the bottom of a sealable container with the onion pieces, then place the avocado half cut side-up on top. Seal the container and keep in the fridge. Luckily, because the skin is the only part of the avocado in contact with the onion, the flesh won't take on any flavor. And you can save the onions for later use!
The Olive Oil Method
Brush the avocado half with olive oil (pick one without a strong flavor). The oil will keep the flesh from coming in direct contact with the air, preventing oxidization. After brushing with the oil, store the avocado in an airtight container in the fridge.
The Lemon Juice Method
You can also brush your avocado's flesh with lemon juice -- the citric acid in the lemon juice dramatically slows the browning process. Again, store in an airtight container for extra protection.