As soon as I get a knife I assign an ID Number and mark a small piece of tape with the number, one tape each for the knife, box, and sheath. I use a spread sheet with 12 columns: ID#; Description; Year; Steel; Condition; Sheath/Box (yes or no); Comments; Purchase Date; Cost; Shipping Cost; Total Cost; Name of Seller.
Since I store the knives separately from the boxes and sheaths, it's important to label everything with the ID#. In addition, I have separate spreadsheets for 110s, 300 Series, Fixed Blades, and Miscellaneous Folders.
Also, I try to take photos right away.
With a spreadsheet, you can sort information in a lot of ways.
The important thing with all of this is to do it right away, otherwise, things get out of hand.