Personally I use lemon juice -- not only does it chemically prevent the browning (yay chemistry) but it also makes it taste better.
>The old rub-with-oil-and-place-face-down-on-an-oiled-plate works fine if you've
>got a perfect half of an avocado with a smooth face, but it doesn't help if
>you've got, say, 3/4 or 1/4 of an avocado.
To save half an avocado for later in the day, I just leave the half with the pit sitting face up. Then later I scrape the tiniest layer of brown off.
Remove the hard bit where the stem attaches (if present.)
With a sharp knife slice into the avocado until you hit the pit.
Rotate the Avocado to slice around the pit until you get back to the starting point (and remove the knife.)
Twist the two halves to separate them. The pit will stick in one half.
Chop the pit with the knife (but not enough to cut it in half!) Twist to remove the pit.
Or just search Youtube for "pit avocado Martha Stewart" who demonstrates this better than I can explain it in about 15s.
But my point was that unless you actually slice the pit in half, neither half is going to have a flat face that you can put on a plate of oil or whatever. Rather one half is going to have the pit sticking out, and the other half is going to have a pit shaped indentation that doesn't touch the oil.