It makes me so angry when I cut into an avocado that was too hard to eat on the day I purchased it, and then when I get ready to eat it, it's gone all mushy and black. If you buy them too, I am sure it has happened to you as well.
The first method below is once I just saw on MSN and the other I saw awhile ago but can't remember where. Both should prevent this annoyance from happening to you again.
If the avocado is ripe and you want to store it away to make guac at your party next week—or in six months—start by washing the fruit with the skin still on. Cut it in half, peel the skin off, and wrap the halves separately (without the pit) in plastic wrap or aluminum foil. Next, pop it in a resealable plastic bag and throw it in the freezer—just be sure to write the date on the bag so you don’t forget about it!
You can also store avocados in the freezer once they’ve been puréed. Squeeze some lime or lemon juice on it and store it in a resealable bag or airtight container for storage.
There is a little bit of a catch to storing avocados for this long. Once you take them out of the freezer they won’t be as firm as when you first bought them. Therefore, you won’t be able to cut it into slices to snack on or add to your sandwich, but you can still mash it up for guacamole or for a dressing or spread.
Source: Reader's Digest: Morgan Cutolo
Another idea, which I have to admit that I haven't tried yet, but some people swear that it works, is once your avocado is well wrapped in aluminum foil, set it inside a baking dish and pop it into the oven at 200°F. Check the fruit's progress at 10 minutes (or even sooner depending on its current degree of ripeness), and keep it in until it's perfectly ripe. Together, the heat of the oven and the ethylene gas will work to soften the fruit.
Hope this helps you to not only save money but not to be disappointed the next time you reach for an avocado!