I just bought a portfolio book for my sketches 
from a particular series, because I have a lot of 
them and I enjoy flipping through, seeing the 
different sequence sketches and the development 
of the image. the rest of my sketches are in one 
of those hard plastic cases you get sometimes 
with cels from japan. I'm sure more people store 
them in clumps than in books though.

of course, sketches are far more archival than 
cels and don't get stuck to things, so you can 
definitely store them in clumps without worrying 
about ruining them.

/Mina