I never used to even photograph my quilts. My sister got me to start doing so after a quilt I made for her youngest. But it was my 4 year old niece (who LOVES quilting! I gave her a design wall for her birthday that she loves playing with) who inspired me to start keeping more information. I made her a little book that tells the story of her quilt being made -- fabrics, layout, tracing the applique shapes, construction, marking, quilting -- through Shutterfly; I still need to have it printed (I want to get one pic with her and the quilt together first) but documenting the process has been so wonderful. I'm thinking of doing that for all my quilts so that when people ask either "hey! Can I get one of those?" or "why are your quilts so expensive?", I can show them all the work that goes into it.
I also keep a couple spreadsheets that track my thread inventory & what fabrics I used in which quilt. I find this is especially helpful to do at the beginning so if I run out of something mid-project, it is fairly easy to source more of whatever I need.