Marginal notes. Occasional use of tiny Post-It flag thingies to mark stuff I want to be able to find again. Sometimes I will grab a highlighter and mark the ed

Marginal notes. Occasional use of tiny Post-It flag thingies to mark stuff I want to be able to find again. Sometimes I will grab a highlighter and mark the edges of pages to make it easier to find sections in a reference work. Possibly with short versions of the section names written next to them, possibly with several colors denoting different kinds of information and a color key written on the flyleaf.