My 2-cents: I keep track of every gun purchased on computer spreadsheet. I keep receipts in a file folder if I have a receipt. I record model name, caliber, serial number, barrel length (handguns), finish, year purchased, year manufactured (and source of that information), purchase store or individual name, box, paper work, any significant accessories-ie scopes on rifles or holsters for handguns, filled out Form 4473?, a paragraph description in as purchased condition for handguns. price, and estimated value. That about covers it.
I intend to start taking digital pictures in the near future as well. I keep records of all firearm transactions (bought and sold). I usually ask to see ID when I sell a gun to an individual whom I don't know. Bottom line, I keep lots of records. I periodically print out a hard copy of the the spread sheet file and have it saved on several media (CD, hard drive, discs).