On Keeping a Reading Journal

I started keeping a reading journal six years ago, mostly because I'd realised I could not remember a single thing about a novel I had apparently loved the previous spring. The Goodreads entry said five stars. My memory said nothing at all.
The rules I settled on are simple:
- One page per book, written within a day of finishing.
- No summary. If I need the plot, I can look it up.
- Write down one sentence I want to remember, and why.
- Write down one thing I disagreed with.
The last rule turned out to be the most useful. Disagreeing with a book, even a book I loved, forces me to read more closely. It also makes the journal far more interesting to reread than a list of things I liked.
Paper or app?
Paper, for me. A plain hardback notebook and whatever pen is nearest. I've tried apps; they make it too easy to write too little. There's something about handwriting that slows me down to the speed at which I actually think.
If you try it, give it three months before you decide. The value only shows up when you flip back and find someone who sounds like you, noticing things you'd since forgotten.