If you have a surplus of ripe bananas, or you make a lot of smoothies, keep a sealed bag of peeled and sliced bananas in the freezer ready to use. Adding ice will dilute the flavour of the smoothie.
Don't just throw them in a bag. Lay them out on a cookie sheet and put them in the freezer. That way they won't stick together. Mix them into a blender for a delicious smoothie. Number three ...