I organize by block type, technique, compendium. Block books are arranged from simple to complex- squares,triangles, diamonds, hex, polygons, stars, compass, mixed & Misc. Techniques are organized by quilting motifs, hand quilting, machine quilting, misc. The last category I sort are books that are compilations like block books or history of "*" books or mixed books with lots of photos of interest.
For finding things, I like to use sticky notes that I place so they stick out beyond the top edge of the book.
Mostly I use my library as a jumping off place for ideas so I'm not too concerned if I can't find something. If I liked something that much I would add it to my list of things I'd like to try when I get the opportunity. Otherwise, my feeling is that if I forgot it, it wasn't memorable enough to think about keeping.
Diana