I also keep a lot of notes on paper. After experimenting with a lot of different formats, I'm now using custom-printed spiral notebooks. They're a nicely portable size, have numbered grid-ruled pages, and have nice hard covers so I can write in them while standing up. Surprisingly inexpensive, too. Not much more than standard notebooks of this quality.
I remember you sharing how you used different sizes of Circa - have you switched to something more generic? Just curious - I've been experimenting with Circa and like being able to move different pages around.