TIP: Mask a mask

If you have added a layer mask to a layer and want to experiment with further masking, here's an easy way to preserve the original mask and mask the layer even more: after adding the layer mask, from the Layers palette popup menu choose New Group from Layers. That will put your layer (and mask) into a group (folder) and now you can add a layer mask to the group, thus, "masking the mask"
Try it, it has some pretty interesting possibilities.