I have an ultra-organized friend who keeps a scrap of each fabric attached to a 3x5 card. She records the number of yards, fabric content, amount paid, and stuff like that on each card. She punches a hole in all of the cards and ring mounts them. She takes the cards with her to the fabric store so she can check to see if she has something similar before purchasing more fabric.
I imagine she doesn't have much time to shop, if she's taking the time to make all the cards