This is one I use all the time when making comics. I draw shapes for my panels, then put them on separate layers. Then I click the targeting circle on the right of the layer, and hit the f-key I have assigned to this action. Illustrator then makes a copy of the panel shape for future use, turns the panel shape into a clipping path for the entire layer, locks that clipping path, and makes a new layer inside this layer called ‘bg’, which contains a slightly larger copy of the panel shape.
Then I can start making more layers inside the panel’s layer and draw freely, without worrying about anything protruding outside of the panel. When I’m done working on a panel I can close up its layer, which results in a lot less clutter in the layers palette.
When I start doing ‘Drowning City’ I may make a variant of this that creates an opacity mask for the layer instead of a clipping mask, so I can draw panels with pseudo-brushed edges. I’m not entirely sure of this yet; I need to see how painted panels look with crisp edges and messy ones.