Refrigerate it. Cold slows the browning enzyme more than anything else you can do, and the lemon juice and the plastic wrap pressed flat against the flesh are the assists, not the main event. Most people do this backwards: they squeeze lemon on it and leave it on the counter, which is the weakest of the three moves without the strongest.
What to check
Press the wrap directly onto the cut face so there is no air gap, then refrigerate. Acid on the surface first, lemon, lime or a little vinegar, slows the reaction further.
The trick worth knowingif you only need half, do not cut the whole thing open and store a naked half. Cut it, twist it apart, then score and scoop the flesh out of one half through its own skin, leaving the other half completely intact with skin and stone in place. That skin is a better oxygen barrier than any wrap you own.
Keeping the stone in place only shields the flesh it actually covers, so it is no miracle, but that patch does stay green.
Browning is cosmetic, not spoilage. Scrape the discolored layer off with the back of a spoon and what is underneath is fine, as long as it still smells like avocado. A sour or fermented smell, or flesh that is slimy or has gray-green fuzz, is not oxidation and it goes in the bin.
Common mistakestoring cut avocado submerged in water. It looks like it works, but the US Food and Drug Administration has warned against it, because bacteria can multiply on the surface during refrigerated storage. It is not a trade worth making.
Written by the team behind RIPE, a free app that reads a photo of the whole produce pile and rings the ripest ones. Every cue above is one you can check with your own hands, and it all works with no app at all.