I usually group by series and then by character, 
like most people.  However, once you get past a 
certain number of cels, this becomes quite 
tiresome, and there is no clear cut easy way to 
organize.  Once I get past a dozen or so cels 
for a series, then I group them additionally by 
cel type.  Is there a background, is the cel a 
pan cel, is the cel a hanken, etc. . .   You can 
imagine how much more messy my cel books are 
compared to the Tenchi section on my site.  Ah 
well, maybe I'll get a helper monkey or 
something to deal with site and cel book 
maintenance.  (I'm in the process right now of 
moving all my cels over to hardcover easel-style 
ring binders. . .very useful, indeed.)