Avocado is particularly troublesome to preserve because it oxidizes so easily (and turns brown). A bit of lemon or lime might help a little, but dealing with a leftover half an avocado is a problem if you've just made a BLTA sandwich.
Fortunately, avocado vacuum seals really well. For best results, leave the pit in (it contains chemicals that also help prevent oxidation) and vacuum seal. This is my half avocado after two days.
Of course you're still always better off using it right away, but in a pinch it saves pretty well in a vacuum, even cut in half.