I divide up my sketch sequences by sequence
number/layer. 

For example, the layout, genga, correction genga,
and final douga for B2 would all be together. I
bag them and put them in the cel book with the
nicest-looking sketch of the bunch on top. 

Then I do the same for the B3 sketches, the B4
sketches, etc. Thus what started as a stack of 30
sketches for a sequence is reduced to five or six
mini-stacks in cel bags, all thin enough to be
displayed in the cel book. 

This way I can still flip through my cel book to
see each step in the sequence, without EVERY
sketch taking up its own page. It also allows me
to hide those sketches that aren't really worth
their own pages to begin with, like mouth layer
corrections. ^_^