photoshop, masks

I'm trying to learn how to use masks. I have this image with 2 layers. I inserted a mask on the top layer. The mask is white, so obviously the entire top layer is visible. I start painting with black to reveal the bottom layer.
     Here is my problem (and question): If I reveal too much of the lower layer, I thought I could just paint with white to cover the lower layer, but when I paint with white, nothing happens. Why? If the mask is white, then painting with black reveals the lower layer and painting with white covers the lower layer again, right? How do I undo the areas that I over-painted with black?