I keep a "Quilt Journal". I use a lined notebook (a pretty covered one if I can find one) and record basically everything I can about the quilt. The pattern name, designer, where found (magazine, online, etc.), fabric choices (sometimes I even add little scraps of fabric to the page) and manufacturer, where fabric purchased, price paid. What prompted me to make that quilt. If it was for a gift, who was it made for and what was the occasion. Any problems I encountered while making the quilt. What I liked/disliked about making the quilt. The date I began and the date I completed the quilt. If I had help making it (friend has mid-arm and will often finish my quilts for me....YES, she is a wonderful friend!!!) I give credit. Finally I take pictures (close-up and distance) of the finished quilt.
I am currently on my second journal and I love looking back and seeing how far I've come as a quilter.
One other thing: I keep a list of "I Want to Make This Quilt" in the journal and as I complete one of these, I enter the date next to it.
Happy Quilting!