If you have kids, chances are pretty good that you have bought bananas for the kids, and then they end up not getting eaten before they start going brown. And if you don't have immediate plans for making baana bread, you can freeze them and use them later in smoothies (or banana bread!)
First, peel bananas. Then dip each banana in lemon juice - this will keep the bananas from turning brown, as they do when exposed to the air. You can chop into chunks and flash freeze (then place in ziploc), or you can freeze them whole in a ziploc bag.
If you are making a smoothie, you can add frozen bananas directly to the blender. If you are using them for baking, defrost before using.