I purchase avocados that are hard, and have not ripened. I let them sit on my counter-top in a paper bag for a day or two - until they get a little soft. Once they are soft, I put them in the fridge. They will ripen on their own, and you won't have to worry about wasting avocados! I swear by this method - I have cut into an avocado that's over a week old and it still looks perfect, and green!
Also, don't forget the save the seed! You can't grow an avocado tree!