Use a plastic bag you get from buying groceries to wrap tightly around the stalk or joint where all the bananas join. Make it as air tight as possible. This will slow the ripening of your bananas. It won't make them last forever but it will give you a few more days before you need to make that Banana Bread.
Best thing about bananas when they go bad just make banana bread the browner the better.
(Yes, I know this is probably a joke, but I'm sure there's some people who really think this way).