I keep one. I'm not as vigilant now with it as I used to be. I would write down each technique I learned in as much detail as I could remember after each class. When I was a white belt, if I couldn't remember all of the finer details, I would talk them out once w/ the coach and then write them out.
I don't do that at regular classes anymore, but at seminars I still do. I LOADED up probably 2-3 pages at a Robert Drysdale seminar. That thing was a treasure trove.
About a year ago, I gathered up all of my journals; scrap pages tucked into books; and wadded up in the bottom of my gi bag, and I typed them all into an Excel spreadsheet (pages organized by category - guard sweeps/attacks, guard passes, mount escapes, mount attacks, takedowns, TD Defense, etc.)
It took me a couple weeks of spare time to do that (a half-hour here, 20 minutes there), but it was amazing running across stuff that I never use in rolling! I'd come in to roll and try to use setups I hadn't thought about for 2-3 years.
I highly recommend doing that.