I go through about one unlined pocket-sized Moleskine per year. Portability trumps all else. I agree that append-only is the way to go; I put a table of contents at the end of each notebook and a header line with the date at the top of each page. (I use the dates rather than page numbers in the table of contents.) I've been doing this since about 2005. Everything goes into the same notebook: notes on "virtual virtual memory" (2020-01-28), notes on foreign exchange rates here in Argentina (2020-01-28), notes on prices of laptops in the store (2020-01-28), a list of things to fix (toilet, clothing, remote control) (2020-01-30), designs for flexural dial indicators (2020-02-01), an explanation of Cantor's diagonalization proof (2020-02-06), thoughts on multitouch UX design (202-02-06), etc. I don't write anything private in the notebook, so I can show it to people. (For the same reason, I write it in Spanish instead of English.)

In order to have space for later commentary, I initially write the notes only on the right-hand pages — the left-hand pages stay blank, then get more stuff added to them later, which may be references to other notes I added later, extensions of the idea, or sometimes just random notes that don't seem to deserve their own note.

I initially wrote in rollerball, but I have the notebook with me all day every day; after a few unfortunate incidents with rain and spilled drinks, I switched to only waterproof markings. I'm using an 0.3mm mechanical pencil, which is my favorite, but I use plain ballpoint pens when that's all that's available. Their ink is oil-based, so it resists water, though not dry alcohol or grease. I can write much smaller with the pencil, so the notebook lasts longer.

I did try using a local brand here in Argentina that apes the Moleskine, called BRÜGGE. I greatly regret it. The cover came unstuck from the notebook after less than a year — apparently the shitheads used a pressure-sensitive adhesive like the one in Scotch Tape rather than traditional bookbinding adhesives like wheat paste or hide glue. A few years later, my girlfriend admired the BRÜGGE notebooks in a store; I warned her, but she wanted one. The same thing happened to her, so it wasn't just a temporary quality mixup — the company is really targeting that abysmal level of quality! Unfortunately BRÜGGE is displacing Moleskine from more and more stationery shops here. (I got mine from Librería Artística Catalinas for US$16; I think they have relocated now to Córdoba 746, Capital Federal. That's also where I got the mechanical pencil, although they didn't have any leads harder than HB.)

Lately I haven't been keeping my address book in the notebook, but when I did, I used a hash table with separate chaining; I numbered the lines in the address-book section, and had a "successor" column. At the beginning of the address book was the hash table itself, a matrix whose rows are groups of first letters (of the person's name) and whose columns are groups of third letters; so "María" whoever would end up in row LM, column QR. If she was the first person in that cell, I'd just write the line number there, but if not, I would add her as a successor at the end of the chain. Difficult to explain (especially to non-programmers) but easy to update and search. Insertion is not worst-case constant time because you can't insert at the beginning of the chain — the paper is WORM rather than strictly read/write.

Here's a sample page, in Spanish, photographed at 5 megapixels: https://is.gd/nbshot

Notes on my computerized notetaking practices are at https://news.ycombinator.com/item?id=23388381