A useful library is easy to search and safe to reorganize. Give files and collections names that describe their content rather than the project where they were first used.
Separate source files from working scenes. Keep the original .blend, then make a project copy when you need to change geometry or materials. Avoid scattering linked dependencies across downloads and temporary directories.
In Blender, mark reusable objects, collections or materials as assets and add the library location in preferences. Catalogs in the Asset Browser help group them by function: furniture, foliage, lighting or kit parts.
Use a few consistent tags. Shape, purpose, style and technical features usually provide more value than a long list of near-synonyms. For large collections, keep a small selection of frequently used resources close at hand.
A preview should show what makes an asset useful. Use a consistent scale and simple lighting for your own library thumbnails.