This one sounds crazy, but bear with me. My grandma swore by it, and grandmas are basically kitchen wizards.
After you make your guac, pour a thin layer of cold water over the top. Not too much—just enough to cover the surface completely.
The water sits on top, blocking all oxygen. When you’re ready to serve, simply pour off the water and give the guac a quick stir. You’ll have bright, green magic underneath.