No sorcery, just science.
I usually just put them in my fridge, and they last a very long time. Mind you, my fridge is barely above freezing.
I find I have no problem if I buy them when they're rock hard and let them soften on my counter (or in my fridge if I want them to take a long time to ripen). However, if I buy them when they're already soft (not overripe) they are all full of black spots and look disgusting. Does anyone know why this is? I've ended up throwing out so many avocados which is so frustrating.
There is a trick for keeping avocados from going brown: if you cut an avocado and don't finish it put it in as small a container as possible with cut onion. It will stay green and purdy much longer. I am not sure how long I eat my avocados fast.
If you are saving a half seal it up with something trying to trap as little air as possible against the exposed avocado flesh. Rubbing lime juice on the exposed flesh can also prolong oxidation.
Why not make a guacamole (loaded with apple cider vinegar or lime juice, plus salt and cilantro/coriander) and leave it in the fridge?