Cut avocado browns because of oxidation: once the flesh meets air, it starts changing color. That means the main job is not to ‘preserve’ it forever, but to slow down contact with oxygen as much as possible. In practice, a few methods genuinely help, while others are mostly kitchen folklore.
What actually works
The most useful tool is acidity. A light coating of lemon or lime juice slows browning better than almost any other home trick because it changes the surface environment and buys you extra time. Use just enough to coat the cut flesh evenly, especially the exposed edges. Too much juice can make the avocado taste sharp, so think of it as a thin layer, not a marinade.
Wraping matters too, but only if it is done tightly. Press plastic wrap directly onto the cut surface so there is as little air gap as possible. If you leave space between the wrap and the avocado, you are mostly just trapping air around it, which defeats the point. An airtight container can help as well, but the key is still reducing air exposure at the surface.
Temperature is the other big factor. A cut avocado keeps its color best in the refrigerator, not on the counter. Cold slows the browning reaction, even if it does not stop it completely. If you only need to hold it for a short time before serving, refrigeration plus a snug cover is the best simple combo.
What helps a little, but not much
Keeping the pit in the half avocado is one of the most overrated tricks. It does protect the small area directly under the seed, but the rest of the exposed flesh still browns. Use it if you want, but do not count on it as a preservation method. It is more of a partial shield than a real solution.
Leaving the skin on the unused half is useful, but only because it reduces exposed surface area. That is why a whole cut half stays better than mashed avocado or slices. The less cut surface you have, the less room there is for oxidation to work.
What is mostly useless
Plain water on the surface is not a reliable fix, and neither is a vague ‘sealed in a bowl’ approach that still leaves lots of air inside. Oil can slow drying, but it does not stop browning in a meaningful way. And while some people swear by onion in the container, that effect is inconsistent and not worth building a whole routine around.
The simplest honest ranking is this: acid plus tight wrap plus refrigeration gives the best results at home. If you want the avocado to look good the next day, slice only what you need, treat the cut surface immediately, and store it cold with minimal air exposure. Everything else is secondary.