I do it the same way John, on an Excel spreadsheet. I just list all of the figures, including the upcoming figures as they're announced, with numerous cells following each one, where I fill in how many I have of said figure - including carded and loose, and how many I still need. I have additional columns for movie, cardback, pack-in, etc. Gotta stick with it, or you can lose track really quickly!
I think there's a couple sites out there that run figure collecting databases, but I always get an uneasy feeling that they may not be as reliable as me tracking my own collecting on my own computer. Just my opinion though...