Freezing them whole didn't work, and I don't have a food dehydrator...yet. So I cut them into slices, arrange them on a plate, and stick them in the freezer. When they're frozen, I transfer them to a freezer bag (or container). My favorite banana bread recipe calls for 3 mashed bananas, so I try to keep them three bananas to a bag. The frozen slices thaw quickly when I'm ready to bake, and they work great for using in smoothies, too!
(Have you discovered Kitchen Tip Tuesdays hosted by TammysRecipes.com yet?!? Go visit!)