I have an undying love for guacamole and all things avocado. (Except for this one weird cold avocado and black bean soup I once ate; that was disgusting.) Anywho, when picking avocados you have either 2 options: buy early or pray to the grocery gods. There is no in between.
When I know I have a special occasion in which success is determined by a bowl of guac I pre-shop. When preshopping you must buy at least 3 days in advance. Look for firm, dark green, unblemished skins. (Oh, and these are haas we're talking about here.) Caring for your nutritionally-rich flavor bombs starts when shopping. If you're bagging your own groceries, be sure to put them on top of the bag. If you're shopping fancy or live anywhere else besides stupid nyc and their lazy grocery employees (excluding Trader Joe's, love you guys!) you need to make sure whomever is bagging up your food doesn't stash your babies at the bottom of a bag. Think of avocados as Fabergé eggs. Treat them with respect.
When you get your lovelies home, assemble your storage device. You will need a large brown paper bag, a fast-food cup holding device, a chip-clip and another fruit, preferably an apple or even better, a banana. Place the cup holder inside the bag and fill the holes with the avocados. In an open spot place your fruit; or if you have 4, use the middle area. The idea is that the avocados do not touch any of their friends or the other fruit. Close up the opening of the bag and secure with chip clip. Check your your fatty fruits once a day until they're needed. Three days is usually the time it takes to get them ripe and ready to eat.
If you have a last minute guacamole emergency, and you know you've all had one, you'll need to go to a very reputable grocer to find ripe ones. When picking ripe ones you should look for tender but not mushy feel. They should give a little but not squish. The skin should be very dark green-brown almost black in color, but play close attention to bruises, mushy areas or dark spots. I have only found one place near me that has these rare specimens on hand and that is the Manhattan Fruit Exchange in Chelsea Market.
I hope all that has help in some way or another. Happy avocadoing!
PS. Dancing avocado gif!