Everyone knows: To keep a cut avocado green and fresh, just leave the pit in the cut half!
Except that everyone knows it doesn't work. Funny how something so easily disproven still floats around as kitchen advice.
So, what does work? Epicurious did some tests on some methods I've used and a few I haven't. On the no side: The pit, plastic wrap, lemon juice, and one that sounded smart to me but doesn't work, non-stick spray.
(By the way, plastic wrap does work if you use wrap that's not oxygen-permeable, such as Saran Wrap Premium.)
So what did work? "Combining lemon juice and plastic wrap was by far the winning solution."
And if you're making guacamole or cutting the avocado, just use the method that kept the Titanic fresh for 73 years. Place it in a bowl and cover it with water.
For guacamole, the national Super Bowl super snack, mash it, press to remove captured air, add 1/2" of water and refrigerate. The next day, pour off and mix.