How to preserve avocados longer

The California Avocado Commission suggestions:

• To ripen a California Avocado, place the fruit in a plain brown paper bag and store at room temperature 65-75° until ready to eat (usually two to five days).
• Including an apple or banana in the bag accelerates the process because these fruits give off ethylene gas, a ripening reagent.
• Soft ripe fruit can be refrigerated until it is eaten, but not for more than two or three days.
• The California Avocado Commission does not recommend using a microwave to accelerate the ripening process.

Depends on if you'll buy for instant consumption or for storage.

If you want to eat/prepare them right away, you want an avocado that feels soft on the inside, when pressing them they will cede and have that soft feeling ripe avocado has. It shouldn't cede very much though, as those are past their prime. The very good ones even smell in a nice avocadoey way a bit (or maybe that's just my insanity.)

If you want to prepare them later, you want firm ones that cede only a tiny little bit (the rock hard ones mostly never ripen in my experience,) you then store them in a porous bag (the typical brown paper bag or a newspaper) to let them ripe. To speed up the ripening, you can add an apple to the bag.

See also this related question