Usually I have a folder named like each client with subfolder /textures and also subfolders for each month Clientname/may2014. After I finished projects for him, I put it onto an archive drive. Textures stay where they are because I reuse them often.
For your car thread here I have a folder 3D-Models/cars and have the converted cars in there. It stay all the time there and the xref uses those texture paths. The whole 3D library is organized this way. For furniture I have subfolders like chairs, tables and so on. Sometimes also for the specific vendors I bought the packs from. After some years it got a little messed up now.
In general for ready-made octane materials I have a Octane-Library.mat file in 3dsmax with most commonly stuff in it. Like some HDRI presets, and other stuff I uses regulary.