I discovered this little gem of a time saver completely by accident. Thanks to a former co-worker of mine I knew about the benefits of Alt+ dragging a layer’s layer effect icon to copy that layer’s style onto another layer.
But what I didn’t know was that you could do the same for the layer’s mask. Just make sure the cursor’s over the layer mask icon.
Of course it makes sense to me now since Alt+dragging almost anywhere in Photoshop means to copy while dragging.
e.g. in the Gradient Map sliders
Layers and layer groups
Pretty much anything you can drag, try Alt+dragging and see if it works as a time-saving copier.