I bought a little plastic photo album at the dollar store and bought some file cards the same size as the openings.
Now when I am collecting fabrics for a project I cut small samples and use doublesided scotch tape to attach them to the file card.
Once the project is completed or the fabric is all used up I can take the samples off the cards and start over!
I also add paint chips of the colours in my house, upholstery samples and curtain samples.