I also use an Excel spreadsheet. I enter the name, system, condition, date of purchase, purchase price, shipping costs, customs, total price and special features and have a hyperlink to an offline version of the Ebay website and photos.
Probably typical German
I have almost similar excel spreadsheet like ichich, but I also try to collect and write down some specific information for every box/item I have. I`m interested to know who were the previous owners of the box/item (if possible) and how, when and where the product was originally on sale etc...
For example I bought a while ago two Thimbleweed Park Limited Run Vinyl versions and they were sold out in couple of days so I add that kind of information to my bookkeeping too
I've started tracking my collection via spreadsheet and similarly do the same as Ahenobarbus; where I try to learn the history of an item - where I purchased it and when (with photos of the sale item), if the seller was the original owner, what kind of environment it was kept in, etc. I'm a bit of a data guy, so this kind of info not only helps me remember and track but really helps give the collection more of a personality.