It's funny that you've posted this, because I'm 
in the middle of re-scanning all my sketches.  My 
problem with it is that the scanned images always 
turn out too light (just the nature of the pencil 
lines), and it's hard to distinguish the image in 
the smaller thumbnails. 

I use the Presto Page Manager program, and it's 
really slow (does *anybody* else use this?), but 
it has the "auto-contrast" and "color correction" 
options to check before scanning.  Using both of 
those makes the scan turn out much better. I 
don't know if you guys can use that, but it's a 
great tool if you have it.  My guess is that it 
might help with brownish-looking scans, as well.

Good luck! 
-Anne