I have a little $2 photo album from Dollar General. I put in a photo of the finished quilt, and of any details I particularly am proud of/want to remember (I give a lot of my quilts away). Then I put in an index card, either behind the photo or on the facing page, that has anything I can remember about the quilt: when I started, when I finished, where the pattern came from, who it was for, what type of batting, binding, etc. was used.
For the quilt I just designed for my sister's baby, I kept all the diagrams, schematics, measurements, templates, etc. together in one manila envelope so that I can make it again.