The post it size note works for me, but I also pin it with a small length dressmaker pin. I simply put width inches first then length inches (45 X36) - this keeps the numbers less confusing to read - then change the length as I use it. Of course, if I have made a length of grain cut and changed the width, I change that too.
Since I buy a fair amount of fabric from Marden's in Maine, this became a real necessity because they are generous in their measure and add several inches to every yard they cut. For those fabrics, I measure when I get it home and put the info on the label they have stapled on for the register clerk.
Since I don't do a lot of true scrap quilting, large scraps I tend to keep with the original fabric they are cut from and often am able to use them instead of cutting from the original piece.
Always interesting to see different ways we try to stay organized.