I've been scanning all my cels and sketches at 
300dpi.

For previews of cels:
* 2 x soften
* resample to new size

For larger views of cels
* 1 x soften
* resample to 50%

For sketches
* resample to 50%

That seems to be working for me -- the only 
sketch that I've had issue with was from Love 
Hina.  The lines were on one side and it was 
colored on the other: it showed up well when you 
held it up near a light, but barely showed up at 
all when scanned.

However, scanning each side separately, mirroring 
the back & then pasting the side with the lines 
on top resulted in the image below 
(unfortunately, the color still doesn't look 
quite like it does in the sketch...)