I bag all my cels -- without pencils -- before storing them
in my cel books.  I used to use the Treasury size and 
Life size bags that we used to carry at our comic store
but they have been discontinued.  Now, I just order the
re-sealable cel bags from Japan.  Magazine boards
work well as an in-betweener but I find that they really
bulk up my cel books.  Hope this helps.

LinC  ^_^