I usually buy some fairly green and some not so green so they do not all ripen at the same time. Here is another method. Take a piece of cling wrap about the size of your fist and wrap it tightly around the crown.
Each time you remove a banana remove and replace the cling wrap. If done correctly this method should give you another few days before your bananas are too ripe to enjoy. Seems it reduces the ethylene which ripens the fruit faster. It it doesn't work, there is always banana bread.