Best way to save an Avocado

Once an avocado is cut, it gets brown pretty quickly - and I hear the "leave the pit in" thing is a myth. If you want to keep the saved avocado half (or a bowl of guacamole) green, store it with a red onion 

Use big chunks of red onion in your guacamole to keep it fresh.... Lemon Juice is second place... I wonder what would happen if you stored it with both... hmmm...