Some will suggest color coding, but I've never understood it as it limits you to about a dozen topics and it presupposes that you'll be interested in those same topics for decades to the exclusion of others. It wholly lacks flexibility.
I use a card index much like H. Ross Ashby. Start with index cards labeled A-Z, then add topics as you encounter them and add a volume number and page number.
Thus:
C<br /> commonplace books: 1-3, 1-88, 4-67 (see also 'Locke, John')<br /> crickets: 2-45<br /> caviar: 3-22, 3-25 (see also 'eggs')
When you've got a handful of cards for each letter it can be useful to separate things out (a la John Locke) as "CA", "CE", "CI", "CO", "CU" and re-alphabetize to make finding things easier and quicker. At this point it can also be helpful to add tabbed dividers to find the "C" section more quickly. Eventually you may have a single card (or three) with an individual heading for topics you write about frequently. (Naturally you could do a single card for each topic as you start, but it often makes the search process take longer and you'll probably have a lot of lonely, unused cards. It also tends to stifle serendipity and creativity because you're not scanning through your topics as thoroughly or frequently.)
I tend to write index words either in the margins of my commonplace or underline them with a red pencil within the text to make finding things on the page easier upon later search.
You can start small with a recipe card box and eventually move your way up to something more industrial as you need it. There are also lots of options in between.
Indexing can be an art and was also a great science (before Google made everyone lazy), so there are some useful handbooks on the topic below:
- Various. 1951. Progressive Indexing and Filing. 5th ed. New York, NY: Remington Rand Inc. http://archive.org/details/progressiveindex0000varo.
- Kaiser, Julius Otto. 1908. Card System at the Office. London: Vacher and Sons. http://archive.org/details/cardsystematoffi00kaisrich.
- Cadwallader, Laura Hanes, and Sarah Ada Rice. 1932. Principles of Indexing and Filing. Baltimore; Chicago: H.M. Rowe Company.
- Kahn, Gilbert, and C. Theo Yerian. 1955. Progressive Filing. New York: Gregg Pub. Division, McGraw-Hill. http://archive.org/details/progressivefilin00kahn.
- Locke, John (1632-1704). 1706. A New Method of Making Common-Place-Books. London. https://archive.org/details/gu_newmethodmaki00lock/mode/2up.
Other related ideas: https://boffosocko.com/research/zettelkasten-commonplace-books-and-note-taking-collection
reply to u/commonbankpen at https://reddit.com/r/commonplacebook/comments/1syayru/how_do_you_index/