I collect fabrics for a certain quilt and keep them together. (ie: Christmas Log Cabin, Matthew's Florence quilt, Michael's blue quilt, Barby's purple quilt, etc) Some types of fabric, I collect "just because" and I keep those together. (30's repros, Civil War repros, Kansas Troubles, coffee prints, fall, Christmas, etc.)
I keep the wide backings separately, unless I find one that's perfect for one of the projects, in which case it stays with its project. Almost everything is in huge bins and like the other poster, I always want something that's in the bottom bin. :D