Here's my proven method: slice your bananas really thin. I'm talking 1/8 inch thin. Use a mandolin slicer if you have one to achieve really uniform thicknesses. Now it's just a matter of leaving them to dry out for the right amount of time. Mine take almost 4 hours, because I soak them in the juice, so they absorb some of it. Here are some variables that will make this process longer: not using a wire rack and placing them directly on parchment, soaking in juice before baking, and having thicker slices. In fact, thicker slices may not crisp up at all before they start to burn.