Make sure whatever you use is acid-free.
I have a friend who folds her fabric twice (so that it's approx. 11" wide) then folds it around her 6 x 24 ruler. Then she slips the ruler out and stacks the fabric on a shelf.
I bought the plastic organizers from www.polarnotions.com. They are a little pricey at a little over $1 each, but I figure they'll last me a whole lot longer than cardboard. And they slip onto the shelf just like a book.